A — The Letter A in the Encyclopedia

A is the first letter of the modern English and Latin alphabets, and it has stood at the head of the alphabet throughout its recorded history. Its ancestor is the Phoenician letter aleph, whose name meant “ox” and whose shape is thought to derive from an ox-head glyph — turn a capital A upside down and the two horns reappear. The Greeks borrowed the sign for a vowel and renamed it alpha, turning it upright; the Romans took it into Latin, and it passed unchanged into English.

Because it opens the alphabet, A leads a second life as a mark of rank and order — the top grade, the first item in a list, the highest tier of a rating. That symbolism, layered on top of its use as the initial of countless words and names, is why a single letter gathers so many meanings: the ampere and the musical note A, blood group A and the DNA base adenine, ISO A4 paper and an A-list celebrity all live below.

A	Common AbbreviationsBattery Sizes A-size batteryDesignation for an uncommon cylindrical 1.5-volt cell, also called an A cell; distinct from AA and AAA sizes. Science Abbreviations absorbanceConventional symbol for the logarithmic measure of light absorbed by a sample in spectrophotometry. Education Abbreviations academic gradeA high or top passing grade in many educational systems; exact standards and the use of plus or minus modifiers vary. Common Abbreviations acePlaying-card rank represented by the letter A; its value depends on the game. Sports Abbreviations acesStatistics-column abbreviation for aces in tennis and some other racket sports. Science Abbreviations adenineOne-letter nucleic-acid code for adenine, one of the four DNA bases — it pairs with thymine (T), while cytosine (C) pairs with guanine (G); in RNA, uracil (U) replaces thymine. Media AbbreviationsIndia Abbreviations adult certificateRestricted film certificate of India’s Central Board of Film Certification, limiting viewing to adults. Stock Symbols Agilent Technologies, Inc.Active ticker symbol for Agilent Technologies on the New York Stock Exchange. Science Abbreviations alanineOne-letter code for alanine, one of the twenty standard amino acids; its three-letter code is Ala. Common AbbreviationsPhonetic Alphabet AlfaCode word for the letter A in the NATO and ICAO phonetic alphabet — the first of its 26 code words (Alfa, Bravo, Charlie, and so on), spelled with an f so speakers of any language say it correctly. Sports Abbreviations alternate captainLetter displayed on the jersey of an alternate captain in ice hockey. Music Abbreviations altoAbbreviation used for the alto part or voice in vocal scoring and ensemble listings. Science AbbreviationsUnits & Measures ampereSI base unit of electric current, and one of the seven SI base units; the symbol is a capital A, written without a period. Science Abbreviations amplitudeConventional symbol for amplitude in physics and wave notation when the context defines it. Technology Abbreviations anodeTerminal label used for the anode of certain electronic components, including diodes and light-emitting diodes. Common Abbreviations answerCommon label for an answer in answer keys, questionnaires, and question-and-answer formats. Technology Abbreviations aperture-priority modeCamera exposure mode in which the photographer selects the aperture and the camera selects a corresponding shutter speed. Mathematics Abbreviations areaConventional mathematical symbol for the area of a plane figure or surface. Sports Abbreviations assistStatistical credit for helping a teammate score in sports such as basketball, ice hockey, and soccer. Sports Abbreviations assistsStatistical abbreviation for assists in baseball fielding records. Military Abbreviations attackBasic mission designator for United States military attack aircraft, as in A-10. Country Codes AustriaInternational vehicle registration code for Austria under the UN Vienna Convention on Road Traffic. Sports Abbreviations awaySchedule or fixture abbreviation identifying an away game or away team. Medical AbbreviationsBlood Groups blood group AOne of the four ABO blood groups (A, B, AB, and O), marked by A antigens on the red blood cells; Rh status is added separately as positive or negative. Common Abbreviations first in order or seriesGeneral letter designation for the first member, category, option, or version in an ordered series. Finance Abbreviations Fitch A ratingLong-term investment-grade credit rating in Fitch Ratings’ A category. Common Abbreviations Grade ATop quality tier in food-grading systems such as USDA grades for eggs and dairy. Science Abbreviations Helmholtz free energySymbol used for Helmholtz free energy in thermodynamics; F is also widely used. Science Abbreviations magnetic vector potentialStandard physics symbol, usually typeset as a bold or vector A. Science Abbreviations mass numberSymbol for the total number of protons and neutrons in an atomic nucleus. Music Abbreviations musical note ALetter name for the sixth degree of the C-major scale and the note orchestras tune to — A above middle C, fixed at 440 hertz (A440). Science Abbreviations radioactive activitySymbol for the decay rate of a radioactive sample, measured in becquerels. Finance Abbreviations S&P A ratingLong-term investment-grade credit rating indicating strong capacity to meet financial commitments, with greater susceptibility to adverse conditions than higher categories. Automobile Abbreviations unrestricted motorcycle categoryDriving-license category for unrestricted motorcycles in the European and United Kingdom category systems. Medical AbbreviationsVitamins vitamin ALetter designation for the group of fat-soluble retinoids tied to vision, growth, and immune function; one of the four fat-soluble vitamins, with D, E, and K.
a	Science Abbreviations accelerationConventional mechanics symbol for acceleration, often typeset as a vector. Science Abbreviations annumLatin for year; combined with prefixes in ka, Ma, and Ga for geological and astronomical time. Science AbbreviationsUnits & Measures areSymbol for the are, a non-SI unit of area equal to 100 square meters; one hundred ares make one hectare. Science AbbreviationsUnit Prefixes atto-SI prefix denoting a factor of 10⁻¹⁸, two steps below pico- on the descending ladder pico, femto, atto. Science Abbreviations cosmological scale factorStandard symbol for the dimensionless scale factor describing cosmic expansion. Science Abbreviations lattice parameterConventional symbol for a unit-cell edge or lattice constant in crystallography. Science Abbreviations semi-major axisSymbol for the semi-major axis of an ellipse or an orbit. Science Abbreviations semimajor axisStandard orbital symbol for the semimajor axis of an ellipse or orbit. Science Abbreviations specific Helmholtz energyConventional lowercase symbol for Helmholtz energy per unit mass in thermodynamics. Science Abbreviations speed of soundConventional symbol for local speed of sound in aerodynamics and gas dynamics. Science Abbreviations thermodynamic activityEffective concentration of a chemical species in thermodynamics and physical chemistry. Science AbbreviationsUnits & Measures yearUnit symbol for annum or year in scientific and technical writing, combined with prefixes in ka (thousand years), Ma (million), and Ga (billion); the exact year definition must be stated when precision matters.
Å	Science AbbreviationsUnits & Measures angstromUnit symbol for the angstrom, equal to 10⁻¹⁰ meter or 0.1 nanometer — a handy scale for atoms, chemical bonds, and the wavelengths of visible light.
A0	Common AbbreviationsPaper Sizes ISO 216 A0 paper sizeThe base size of the ISO A-series, measuring 841 × 1,189 millimeters — an area of almost exactly one square meter, halved at each step down the series. Music Abbreviations musical pitch A0Scientific pitch notation for A in octave 0; frequency is 27.5 hertz in twelve-tone equal temperament when A4 equals 440 hertz.
A1	Science Abbreviations A1 beta-caseinOne of the principal genetic variants of bovine beta-casein milk protein. Medical AbbreviationsBlood Groups A1 blood subgroupMajor subgroup of blood group A distinguished by stronger A-antigen expression than A2. Automobile Abbreviations category A1 motorcycle licenseLicense category for light motorcycles, generally up to 125 cubic centimeters and 11 kilowatts in European and UK systems. Education Abbreviations CEFR A1Breakthrough or beginner proficiency level in the Common European Framework of Reference for Languages. Common Abbreviations first-rate or excellentInformal designation for first-class quality, from the Lloyd’s Register classification of ships. Common AbbreviationsPaper Sizes ISO 216 A1 paper sizeInternational A-series paper format measuring 594 × 841 millimeters (23.4 × 33.1 inches). Finance Abbreviations Moody’s A1 ratingUpper-medium-grade long-term credit rating at the higher end of Moody’s A category. Music Abbreviations musical pitch A1Scientific pitch notation for A in octave 1; frequency is 55 hertz in twelve-tone equal temperament when A4 equals 440 hertz. Science AbbreviationsVitamins vitamin A1Retinol, the principal dietary form of vitamin A.
A1 Movers	Education Abbreviations A1 MoversCambridge English young-learners qualification aligned with CEFR level A1.
A1AD	Medical Abbreviations alpha-1 antitrypsin deficiencyInherited condition caused by deficient or dysfunctional alpha-1 antitrypsin; AATD is another common abbreviation.
A1AT	Medical Abbreviations alpha-1 antitrypsinProtective serine-protease inhibitor produced mainly by the liver; also abbreviated AAT.
A1C	Military Abbreviations Airman First ClassEnlisted rank, pay grade E-3, in the United States Air Force and Space Force. Medical Abbreviations hemoglobin A1cGlycated hemoglobin measured to estimate average blood glucose over roughly the preceding two to three months.

A1PI	Medical Abbreviations alpha-1 proteinase inhibitorAlternative name and abbreviation for alpha-1 antitrypsin.
A2	Science Abbreviations A2 beta-caseinOne of the principal genetic variants of bovine beta-casein milk protein. Medical AbbreviationsBlood Groups A2 blood subgroupMajor subgroup of blood group A with lower A-antigen expression than A1. Medical Abbreviations aortic component of the second heart soundCardiology notation for closure of the aortic valve as part of S2. Automobile Abbreviations category A2 motorcycle licenseLicense category for medium-power motorcycles, generally up to 35 kilowatts in European and UK systems. Education Abbreviations CEFR A2Waystage or elementary proficiency level in the Common European Framework of Reference for Languages. Common AbbreviationsPaper Sizes ISO 216 A2 paper sizeInternational A-series paper format measuring 420 × 594 millimeters (16.5 × 23.4 inches). Finance Abbreviations Moody’s A2 ratingUpper-medium-grade long-term credit rating in the middle of Moody’s A category. Music Abbreviations musical pitch A2Scientific pitch notation for A in octave 2; frequency is 110 hertz in twelve-tone equal temperament when A4 equals 440 hertz. Science AbbreviationsVitamins vitamin A23,4-didehydroretinol, a form of vitamin A found chiefly in freshwater fish.
A2 Flyers	Education Abbreviations A2 FlyersCambridge English young-learners qualification aligned with CEFR level A2.
A2 Key	Education Abbreviations A2 KeyCambridge English qualification aligned principally with CEFR level A2; formerly known as the Key English Test.
A2 Key for Schools	Education Abbreviations A2 Key for SchoolsCambridge English qualification aligned with CEFR level A2 and designed for school-age learners.
A2A	Finance Abbreviations account-to-accountPayment transfer made directly between bank accounts rather than through a card network. Military Abbreviations air-to-airAlphanumeric shorthand for operations, weapons, communications, or engagements between airborne platforms. Technology Abbreviations application-to-applicationData exchange or integration directly between software applications.
A2DP	Technology Abbreviations Advanced Audio Distribution ProfileBluetooth profile for high-quality audio streaming from a source device to a receiving device.
A2G	Military Abbreviations air-to-groundAlphanumeric shorthand for communication, weapons, or operations from an airborne platform to the ground.
A2P	Technology Abbreviations application-to-personMessaging traffic sent from a software application to an individual user, commonly by SMS or rich messaging.
A3	Business Abbreviations A3 problem-solving methodLean-management method that documents a problem, analysis, countermeasures, and follow-up on an A3-size report. Business Abbreviations A3 reportStructured one-page problem-solving format used in lean management, named for the A3 paper size. Common AbbreviationsPaper Sizes ISO 216 A3 paper sizeInternational A-series paper format measuring 297 × 420 millimeters (11.7 × 16.5 inches). Finance Abbreviations Moody’s A3 ratingUpper-medium-grade long-term credit rating at the lower end of Moody’s A category. Music Abbreviations musical pitch A3Scientific pitch notation for A in octave 3; frequency is 220 hertz in twelve-tone equal temperament when A4 equals 440 hertz.
A4	Music Abbreviations A above middle CThe pitch A4 in scientific pitch notation, the standard tuning reference set at 440 hertz. Common AbbreviationsPaper Sizes ISO 216 A4 paper sizeInternational A-series paper format measuring 210 × 297 millimeters (8.27 × 11.7 inches); the most widely used ISO 216 size, reached by halving A0 four times. Music Abbreviations musical pitch A4Scientific pitch notation for A in octave 4; frequency is 440 hertz in twelve-tone equal temperament when A4 equals 440 hertz.
A5	Common AbbreviationsPaper Sizes ISO 216 A5 paper sizeInternational A-series paper format measuring 148 × 210 millimeters (5.83 × 8.27 inches). Music Abbreviations musical pitch A5Scientific pitch notation for A in octave 5; frequency is 880 hertz in twelve-tone equal temperament when A4 equals 440 hertz.
A6	Common AbbreviationsPaper Sizes ISO 216 A6 paper sizeInternational A-series paper format measuring 105 × 148 millimeters (4.13 × 5.83 inches). Music Abbreviations musical pitch A6Scientific pitch notation for A in octave 6; frequency is 1,760 hertz in twelve-tone equal temperament when A4 equals 440 hertz.
A7	Common AbbreviationsPaper Sizes ISO 216 A7 paper sizeInternational A-series paper format measuring 74 × 105 millimeters (2.91 × 4.13 inches). Music Abbreviations musical pitch A7Scientific pitch notation for A in octave 7; frequency is 3,520 hertz in twelve-tone equal temperament when A4 equals 440 hertz.
A8	Common AbbreviationsPaper Sizes ISO 216 A8 paper sizeInternational A-series paper format measuring 52 × 74 millimeters (2.05 × 2.91 inches). Music Abbreviations musical pitch A8Scientific pitch notation for A in octave 8; frequency is 7,040 hertz in twelve-tone equal temperament when A4 equals 440 hertz.
A9	Common AbbreviationsPaper Sizes ISO 216 A9 paper sizeInternational A-series paper format measuring 37 × 52 millimeters (1.46 × 2.05 inches). Music Abbreviations musical pitch A9Scientific pitch notation for A in octave 9; frequency is 14,080 hertz in twelve-tone equal temperament when A4 equals 440 hertz.
A10	Common AbbreviationsPaper Sizes ISO 216 A10 paper sizeInternational A-series paper format measuring 26 × 37 millimeters (1.02 × 1.46 inches).
a11y	Technology Abbreviations accessibilityNumeronym formed from the first and last letters of accessibility with 11 omitted letters between them.
A23	Common AbbreviationsBattery Sizes A23 batteryCompact cylindrical 12-volt alkaline battery used in keyless-entry transmitters, doorbells, and similar devices; also known by several manufacturer and IEC equivalents.
A24	Media Abbreviations A24Current official name of an American independent film and television production and distribution company.
A27	Common AbbreviationsBattery Sizes A27 batterySlim cylindrical 12-volt alkaline battery used in small remote controls and security transmitters; commonly associated with the 8LR732 designation.
A220	Aviation Abbreviations Airbus A220Current narrow-body commercial-aircraft family originally developed as the Bombardier CSeries.
A260	Science Abbreviations absorbance at 260 nanometersSpectrophotometric measurement commonly used to estimate nucleic-acid concentration.
A260/A280	Science Abbreviations 260-to-280-nanometer absorbance ratioSpectrophotometric purity indicator used primarily for nucleic-acid samples.
A280	Science Abbreviations absorbance at 280 nanometersSpectrophotometric measurement commonly used to estimate protein concentration and assess nucleic-acid purity.
A320	Aviation Abbreviations Airbus A320Current narrow-body commercial-aircraft family that includes A319, A320, and A321 variants.
A330	Aviation Abbreviations Airbus A330Current wide-body twin-engine commercial-aircraft family, including passenger, freighter, and tanker-derived versions.
A350	Aviation Abbreviations Airbus A350Current long-range wide-body twin-engine commercial-aircraft family.
A380	Aviation Abbreviations Airbus A380Large double-deck wide-body commercial-aircraft type that remains in active airline service.
A400M	Military Abbreviations Airbus A400M AtlasActive four-engine turboprop military transport aircraft produced by Airbus Defence and Space.
A440	Music Abbreviations A440 concert pitchInternational tuning standard (ISO 16) fixing the pitch A above middle C at 440 hertz — the note orchestras tune to. Music Abbreviations A440 pitch standardTuning standard setting musical A4 above middle C to exactly 440 hertz; standardized by ISO 16.
A antigen	Medical Abbreviations blood-group A antigenCell-surface carbohydrate antigen that defines blood group A in the ABO system.
A coefficient	Science Abbreviations Einstein A coefficientCoefficient giving the probability per unit time of spontaneous emission from an excited atomic or molecular state.
A cup	Common AbbreviationsBra Sizes A-cup bra sizeSmallest of the standard bra cup letters (A, B, C, D, and up); the cup reflects the difference between bust and band measurement, so its actual volume varies with band size and sizing system.
A h	Science AbbreviationsUnits & Measures ampere hourUnit of electric charge commonly used for battery capacity; also written A·h or Ah, with Ah reserved for the AH batch.
A horizon	Science Abbreviations A soil horizonMineral soil horizon at or near the surface, commonly enriched with humified organic matter or affected by eluviation.
A level	Education AbbreviationsUK Abbreviations Advanced LevelSubject-based secondary qualification used in the United Kingdom and related educational systems; A-level is common as an attributive form.
A major	Music Abbreviations A majorMajor key whose tonic is A; its key signature has three sharps (F♯, C♯, and G♯).
A minor	Music Abbreviations A minorMinor key whose tonic is A and the relative minor of C major; its natural-minor key signature has no sharps or flats.
A record	Technology Abbreviations address recordDomain Name System resource record that maps a domain name to a 32-bit IPv4 address.
A s	Science AbbreviationsUnits & Measures ampere secondSI coherent unit expression for electric charge, equal to one coulomb; also written A·s.
A series	Common AbbreviationsPaper Sizes ISO 216 A-series paper sizesInternational paper-size series based on a 1-to-square-root-of-2 aspect ratio and including A0 through A10.
A string	Music Abbreviations A stringInstrument string tuned to the pitch A, such as the second-highest string of a violin or a designated string on a guitar.
A visa	Government Abbreviations United States A visa categoryNonimmigrant visa class for diplomats, foreign-government officials, qualifying employees, and certain immediate family members; subdivided into A-1, A-2, and A-3.
A$	Finance Abbreviations Australian dollarCommon distinguishing currency symbol for the Australian dollar; the active ISO 4217 code is AUD.
A&B	Law Abbreviations assault and batteryCriminal charge combining the threat of force with unlawful physical contact.

A&D	Business Abbreviations aerospace and defenseIndustry abbreviation covering commercial aerospace, defense manufacturing, and related services.
A&E	Media AbbreviationsUSA Abbreviations A&ECurrent name of the United States television network whose brand originated from Arts & Entertainment Network. Medical AbbreviationsUK Abbreviations accident and emergencyHospital department for urgent and emergency care in the United Kingdom and Ireland; equivalent to the US emergency room. Business Abbreviations architecture and engineeringCombined professional-services designation used for building and infrastructure design work. Media AbbreviationsUSA Abbreviations Arts & EntertainmentAmerican pay-television network, branded A&E.
A&F	Business Abbreviations Abercrombie & FitchCommon shortened brand form for the American apparel retailer Abercrombie & Fitch.
A&M	Music Abbreviations A&M RecordsAmerican record label founded in 1962 by Herb Alpert and Jerry Moss. Education AbbreviationsUSA Abbreviations agricultural and mechanicalExpansion retained in the historical names of several United States colleges and universities. Education AbbreviationsUSA Abbreviations Texas A&M UniversityCommon short form for the Texas public research university; A&M remains part of the official name and is not currently expanded in the legal name.
A&M Records	Music Abbreviations A&M RecordsRecord-label name originally derived from the surnames Alpert and Moss; it functions as a lexicalized brand.
A&O	Medical Abbreviations alert and orientedClinical shorthand describing a patient’s level of awareness; often followed by the number of orientation domains assessed.
A&P	Aviation Abbreviations airframe and powerplantPaired United States mechanic ratings commonly associated with FAA-certificated aircraft mechanics. Education Abbreviations anatomy and physiologyCommon course-title and subject abbreviation in health and life-science education.
A&R	Music Abbreviations artists and repertoireRecord-label or music-publishing function responsible for talent discovery and artistic development.
A&S	Education Abbreviations arts and sciencesCommon name and abbreviation for an academic college, faculty, or curriculum division.
A&W	Business AbbreviationsUSA Abbreviations A&W RestaurantsCurrent restaurant-chain brand formed from the surnames of founders Roy Allen and Frank Wright.
A*	Education AbbreviationsUK Abbreviations A-star gradeHighest grade, called A-star, at GCSE in Wales and Northern Ireland and at A-level across England, Wales, and Northern Ireland. Technology Abbreviations A-star search algorithmBest-first graph-traversal and pathfinding algorithm widely used in computing. Mathematics Abbreviations adjoint of ALinear-algebra notation for the adjoint or conjugate transpose of a matrix or operator. Technology Abbreviations Kleene closure of AFormal-language notation for all finite concatenations of elements or strings from A, including the empty string.
A*STAR	Government AbbreviationsSingapore Abbreviations Agency for Science, Technology and ResearchSingapore’s lead public-sector agency for scientific and industrial research; styled A*STAR.
A+	Medical AbbreviationsBlood Groups A positiveBlood group A that is RhD positive in the ABO and Rh systems. Education Abbreviations A-plus gradeAcademic grade above A where plus and minus grading is used; the numerical value varies by institution. Education Abbreviations academic gradeGrade above a plain A in systems that use plus and minus modifiers; typically the highest mark. Technology Abbreviations CompTIA A+Vendor-neutral information-technology support certification maintained by CompTIA. Common Abbreviations excellent or superiorInformal evaluative mark indicating exceptionally high quality or performance. Finance Abbreviations Fitch A-plus ratingUpper modifier within Fitch Ratings’ long-term A category. Technology Abbreviations one or more occurrences of ARegular-expression quantifier meaning one or more consecutive occurrences of the preceding token represented here by A. Finance Abbreviations S&P A-plus ratingLong-term investment-grade credit rating one notch above A in the S&P Global Ratings scale.
A+E	Media Abbreviations A+E Global MediaCurrent corporate brand of the media company formerly known as A+E Networks; the name was adopted in 2025.
A-1	Common Abbreviations first-rate or excellentInformal quality designation; also written A1 or A-one. Finance Abbreviations S&P A-1 ratingStrong short-term capacity to meet financial commitments in the S&P Global Ratings scale. Government Abbreviations United States A-1 visaNonimmigrant diplomatic visa category for specified senior foreign-government representatives and immediate family members.
A-1+	Finance Abbreviations S&P A-1-plus ratingHighest designation within S&P’s A-1 short-term category, indicating extremely strong capacity to meet financial commitments.
A-2	Finance Abbreviations S&P A-2 ratingSatisfactory short-term capacity to meet financial commitments, with greater susceptibility to adverse conditions than A-1. Government Abbreviations United States A-2 visaNonimmigrant visa category for other qualifying foreign-government officials, employees, and immediate family members.
A-3	Finance Abbreviations S&P A-3 ratingAdequate short-term capacity to meet financial commitments, vulnerable to adverse economic conditions. Government Abbreviations United States A-3 visaNonimmigrant visa category for attendants, servants, or personal employees of A-1 or A-2 visa holders and their immediate families.
A-10	Military Abbreviations A-10 Thunderbolt IIActive United States Air Force close-air-support and attack aircraft commonly nicknamed the Warthog; its service life was extended through 2030 in 2026.
A-29	Military Abbreviations A-29 Super TucanoActive light-attack and advanced-training military aircraft designation used for the Embraer EMB 314 in several air forces.
A-arm	Automobile Abbreviations A-shaped control armAutomotive suspension link shaped approximately like the letter A; also called a wishbone control arm.
A-band	Medical Abbreviations anisotropic bandDark band of a striated-muscle sarcomere containing the full length of thick myosin filaments; commonly called the A band.
A-bomb	Military Abbreviations atomic bombEstablished shortened form for a nuclear weapon based principally on atomic fission.
A-camera	Media Abbreviations primary cameraThe principal camera assigned to the main angle or most important coverage in a multicamera production.
A-CDM	Aviation Abbreviations airport collaborative decision-makingOperational framework for sharing timely airport information among airport operators, airlines, ground handlers, and air traffic management partners.
A-DNA	Science Abbreviations A-form DNARight-handed double-helical DNA conformation favored by dehydrating conditions and also found in some DNA–RNA hybrids.
A-fib	Medical Abbreviations atrial fibrillationCommon shortened form for a cardiac arrhythmia characterized by disorganized atrial electrical activity; AF and AFib are also used.
A-file	Government Abbreviations alien fileUnited States immigration case file associated with an individual’s Alien Registration Number.
A-flutter	Medical Abbreviations atrial flutterCommon shortened form for a macroreentrant atrial tachyarrhythmia; AFL is also used.
A-Frame	Technology Abbreviations A-FrameOpen-source web framework for creating three-dimensional and virtual-reality experiences using HTML-style markup.
A-frame	Common Abbreviations A-frame buildingBuilding whose steeply sloping roof sides extend near the ground and form an A-shaped profile. Common Abbreviations A-frame structural supportTriangular support or framework shaped like a capital A.
A-game	Common Abbreviations A-gameInformal term for one’s highest level of performance. Common Abbreviations highest level of performanceInformal expression for a person’s best effort, skill, or performance.
A-GPS	Technology Abbreviations assisted Global Positioning SystemPositioning method that uses network-supplied assistance data to reduce satellite-acquisition time or improve availability.
A-hole	Common Abbreviations assholeEuphemistic shortened form of a vulgar insult; informal and potentially offensive.
A-law	Technology Abbreviations A-law compandingLogarithmic audio companding standard used in digital telecommunications, particularly in many countries outside North America and Japan.
A-line	Common Abbreviations A-line silhouetteGarment shape fitted toward the top and widening toward the hem to resemble a capital A. Medical Abbreviations arterial lineCatheter placed in an artery for continuous blood-pressure monitoring or repeated arterial blood sampling. Medical Abbreviations lung-ultrasound A-lineHorizontal reverberation artifact parallel to the pleural line, commonly seen when air is present beneath the pleura.
A-list	Common Abbreviations A-listInformal term for the most sought-after and prominent celebrities or people. Common Abbreviations highest-status groupInformal term for the most prominent, desirable, or influential people or items in a field.
A-mode	Medical Abbreviations amplitude modeOne-dimensional ultrasound display in which echo amplitude is plotted against depth or time.
A-movie	Media Abbreviations A movieMajor, well-financed, or prestige feature film, traditionally contrasted with a lower-budget B movie.
A-MPDU	Technology Abbreviations aggregate MAC protocol data unitIEEE 802.11 frame aggregation method that combines multiple MAC protocol data units for transmission.
A-MSDU	Technology Abbreviations aggregate MAC service data unitIEEE 802.11 frame aggregation method that combines multiple MAC service data units within one MAC protocol data unit.
A-number	Government Abbreviations Alien Registration NumberUnique identifying number assigned by the United States government to many noncitizens; often printed with an A prefix.
A-OK	Common Abbreviations A-OKInformal Americanism meaning in perfect working order; popularized during the United States space program. Common Abbreviations all correctInformal expression meaning completely satisfactory or in good order; the exact origin of the A is disputed.
A-one	Common Abbreviations A-oneInformal term meaning first-rate or excellent; equivalent to A1. Common Abbreviations first-rateInformal adjective meaning excellent or of the highest quality; also written A1.
A-pillar	Automobile Abbreviations A-pillarForemost roof-support pillar on each side of a passenger vehicle, bordering the windshield.
A-plot	Media Abbreviations primary plotlineThe main narrative thread in an episode, screenplay, or other dramatic work.
A-road	Common AbbreviationsUK Abbreviations A-roadMajor numbered road category in Great Britain, below motorways in the road-classification hierarchy.
A-roll	Media Abbreviations primary footagePrincipal footage and synchronized sound carrying the main action or interview in film and video editing.
A-scan	Medical Abbreviations amplitude scanOne-dimensional diagnostic-ultrasound technique, especially associated with ophthalmic measurement.
A-segment	Automobile Abbreviations A-segment passenger carSmallest commonly recognized European passenger-car market segment, often called the mini-car class.
A-share	Finance AbbreviationsChina Abbreviations China A-shareShare of a mainland Chinese company traded in renminbi on a mainland stock exchange and accessible under current market rules.
A-side	Music Abbreviations A-side of a singleSide of a phonograph single designated as the principal or promoted recording.
A-site	Science Abbreviations aminoacyl siteRibosomal binding site that accepts an incoming aminoacyl-transfer RNA during protein synthesis.
A-SMGCS	Aviation Abbreviations advanced surface movement guidance and control systemAirport system providing surveillance, routing, guidance, and control of aircraft and vehicles on the movement area, including in low visibility.
A-story	Media Abbreviations primary storyThe principal narrative or editorial story, distinguished from secondary B- or C-stories.
A-team	Common Abbreviations A-teamInformal term for a group of the most skilled or first-choice members. Common Abbreviations first-choice teamInformal term for the most capable or preferred group assigned to a task.
A-tubule	Science Abbreviations A tubuleComplete microtubule of each outer doublet in cilia and flagella; paired with an incomplete B tubule.
A-type asteroid	Science Abbreviations spectral type A asteroidRare asteroid spectral class characterized by a strong olivine-related absorption feature and a relatively reddish spectrum.
A-type star	Science Abbreviations spectral type A starStar in spectral class A, characterized by strong hydrogen absorption lines and a white to blue-white appearance.
A-value	Science Abbreviations A valueQuantitative measure of a substituent’s preference for an equatorial rather than axial position in a monosubstituted cyclohexane.
A-VDGS	Aviation Abbreviations advanced visual docking guidance systemAutomated stand-guidance system that helps pilots align and stop an aircraft accurately at an airport gate.
A-wave	Medical Abbreviations atrial contraction waveWave produced by atrial contraction, identified in jugular venous pulse or Doppler-flow assessment depending on context.
A-weighting	Science Abbreviations A-frequency weightingStandard sound-level weighting that approximates human hearing sensitivity at moderate levels; measurements are often written dB(A).
A.	Law Abbreviations answerMarker for a deponent’s or witness’s answer in legal transcripts, paired with Q for question. Law Abbreviations Atlantic ReporterRegional case reporter of United States state-court decisions; later series are cited A.2d and A.3d.
a.	Common Abbreviations adjectiveDictionary and grammatical abbreviation for adjective.
A.1.	Business Abbreviations A.1. SauceCurrent brand name for a brown sauce sold in North America by Kraft Heinz and in the United Kingdom by Premier Foods.
A/A	Military Abbreviations air-to-airSlash-form abbreviation for operations, communications, or weapons involving airborne targets.
A/B	Technology Abbreviations A/B switchSelector or switching device that chooses between two signal paths, sources, or destinations labeled A and B. Technology Abbreviations A/B system updatesOperating-system update architecture using two system slots so one can be updated while the other remains bootable. Business Abbreviations A/B testingControlled experiment comparing variant A with variant B to measure a defined outcome.
A/C	Finance Abbreviations accountShorthand for a financial account in banking and bookkeeping. Technology Abbreviations air conditioningCommon abbreviation for air-conditioning equipment or service. Aviation Abbreviations aircraftOperational shorthand for aircraft in aviation and military contexts. Science Abbreviations alternating currentEstablished punctuated variant of AC for electric current that periodically reverses direction. Medical Abbreviations assist-control ventilationMechanical-ventilation mode in which each patient-triggered or time-triggered breath receives a preset level of support.
a/c	Finance Abbreviations accountCommon business and bookkeeping abbreviation, especially in British and Commonwealth usage.
A/D	Technology Abbreviations analog-to-digitalConversion or converter designation for transforming an analog signal into digital data.
A/F	Automobile Abbreviations air–fuel ratioRatio of the mass of air to the mass of fuel in an engine’s combustible mixture.
A/G	Finance Abbreviations administrative and generalExpense-category abbreviation used in utility, insurance, and organizational accounting for administrative and general costs. Aviation Abbreviations air-to-groundDesignation for communication, weapons, or operations between an aircraft and the ground. Medical Abbreviations albumin/globulin ratioLaboratory ratio comparing serum albumin with globulin; also called the albumin/globulin ratio.
A/L	Finance Abbreviations assets and liabilitiesBalance-sheet shorthand referring collectively to assets and liabilities.
A/m	Science AbbreviationsUnits & Measures ampere per meterSI derived unit expression used for magnetic field strength.
A/m²	Science AbbreviationsUnits & Measures ampere per square meterSI derived unit expression used for electric current density.
A/P	Finance Abbreviations accounts payableAmounts a business owes to suppliers or other creditors for goods and services received. Medical Abbreviations assessment and planSection of a clinical note that records the clinician’s evaluation and intended management. Aviation Abbreviations autopilotSystem that automatically controls the flight path of an aircraft.
A/R	Finance Abbreviations accounts receivableAmounts owed to a business by customers or other debtors.
A/S	Finance Abbreviations after sightNotation on a bill of exchange indicating that the payment period begins when the bill is presented. Military Abbreviations air-to-surfaceSlash-form abbreviation for operations or weapons directed from an aircraft toward a surface target. Business Abbreviations alongsideShipping term indicating that goods are delivered next to a vessel.
A/S/L	Texting Abbreviations age, sex, locationEarly online-chat request asking a person’s age, sex, and location; now often recognized as dated Internet slang.
A/T	Automobile Abbreviations all-terrainDesignation for tires suited to both on-road and off-road use. Military Abbreviations anti-tankMilitary modifier for weapons, units, and operations intended to defeat armored vehicles. Automobile Abbreviations automatic transmissionAutomotive abbreviation for a transmission that changes gear ratios automatically. Aviation Abbreviations autothrottleAircraft system that automatically controls engine thrust or power to maintain selected targets.
A/V	Technology Abbreviations audiovisualRelating to combined audio and visual equipment, media, or services.
A/W	Science AbbreviationsUnits & Measures ampere per wattUnit of electrical responsivity, especially for photodetectors. Business Abbreviations autumn/winterFashion-industry season designation for autumn and winter collections; also styled AW.
A?	Technology Abbreviations zero or one occurrence of ARegular-expression quantifier meaning that the preceding token represented here by A is optional.
A°	Mathematics Abbreviations interior of ATopology notation for the set of interior points of A; a small circle is written as a superscript.
A·m²	Science AbbreviationsUnits & Measures ampere square meterSI derived unit expression commonly used for magnetic dipole moment.
Aα	Medical Abbreviations A-alpha nerve fibersLarge, heavily myelinated group A sensory and motor nerve fibers with the fastest conduction velocities.
Aβ	Medical Abbreviations A-beta nerve fibersMyelinated group A sensory fibers associated especially with touch, pressure, and proprioception. Medical Abbreviations amyloid betaPeptide commonly abbreviated Aβ and associated with amyloid plaques in Alzheimer disease.
Aγ	Medical Abbreviations A-gamma nerve fibersMyelinated motor fibers supplying intrafusal muscle fibers in muscle spindles.
Aδ	Medical Abbreviations A-delta nerve fibersThinly myelinated sensory fibers associated with fast pain, cold, and some touch sensations.
Aᵀ	Mathematics Abbreviations transpose of AStandard linear-algebra notation for the transpose of a matrix A.
Aᶜ	Mathematics Abbreviations complement of ACommon set-theory and probability notation for all outcomes or elements outside A, relative to the relevant universal set.
A–a	Medical Abbreviations alveolar–arterial oxygen gradientDifference between alveolar and arterial oxygen partial pressures; also written A-a gradient.
A–T	Science Abbreviations adenine–thymine base pairComplementary DNA base pair joined by two hydrogen bonds.
A–U	Science Abbreviations adenine–uracil base pairComplementary RNA base pair joined by two hydrogen bonds.
A⁺	Mathematics Abbreviations Moore–Penrose pseudoinverse of ACommon linear-algebra notation for the Moore–Penrose inverse. Technology Abbreviations positive closure of AFormal-language notation for one or more concatenations from A; unlike A*, it excludes the empty string unless independently generated.
A⁻¹	Mathematics Abbreviations inverse of AStandard notation for the inverse of an invertible matrix, function, or algebraic element, as determined by context.
a₀	Science Abbreviations Bohr radiusPhysical constant for the radius of the ground state of the hydrogen atom, about 0.529 angstrom.
A₁	Mathematics Abbreviations first coefficientConventional subscript notation for the first term or coefficient in a sequence or series. Mathematics Abbreviations first indexed member of AStandard subscript notation for the first term, component, coefficient, or member of an indexed family.
Aₙ	Mathematics Abbreviations alternating group of degree nStandard group-theory notation for the group of even permutations on n symbols. Mathematics Abbreviations nth indexed member of AStandard notation for the nth term, component, coefficient, or member of an indexed family. Mathematics Abbreviations type-A root system or Lie algebraStandard classification notation in Lie theory and the theory of Dynkin diagrams.
A−	Medical AbbreviationsBlood Groups A negativeBlood group A that is RhD negative in the ABO and Rh systems. Education Abbreviations A-minus gradeAcademic grade below A where plus and minus grading is used; the numerical value varies by institution. Education Abbreviations academic gradeGrade just below a plain A in systems that use plus and minus modifiers. Finance Abbreviations Fitch A-minus ratingLower modifier within Fitch Ratings’ long-term A category. Finance Abbreviations S&P A-minus ratingLong-term investment-grade credit rating one notch below A in the S&P Global Ratings scale; commonly typed A-.
A⊥	Mathematics Abbreviations orthogonal complement of ALinear-algebra and functional-analysis notation for vectors orthogonal to every vector in A.
A♭	Music Abbreviations A-flatMusical pitch one chromatic semitone below A in standard Western notation.
A♮	Music Abbreviations A-naturalMusical A with a natural sign, canceling a previous sharp or flat.
A♯	Music Abbreviations A-sharpMusical pitch one chromatic semitone above A in standard Western notation.
A𝄪	Music Abbreviations A-double-sharpMusical pitch two chromatic semitones above A in standard notation.
A𝄫	Music Abbreviations A-double-flatMusical pitch two chromatic semitones below A in standard notation.

Frequently asked questions

What does A stand for?

A has dozens of meanings depending on context. As a symbol it is the ampere (electric current), the musical note A, blood group A, and the DNA base adenine; as a short form it stands for words from “answer” to “Austria.” This band of the encyclopedia lists 293 distinct meanings of A and its close variants, each with its field.

What is the SI unit symbol A?

The uppercase A is the symbol for the ampere, the SI base unit of electric current, written without a period. The lowercase a is a separate symbol — for acceleration in physics, the are in area, or the SI prefix atto- (10⁻¹⁸).

What is the note A in music?

A is the sixth degree of the C major scale and the pitch orchestras tune to — A above middle C, standardized at 440 Hz (written A4 or A440). A major has three sharps; A minor, the relative minor of C major, has none.

What size is A4 paper?

A4 measures 210 x 297 mm and is the most widely used ISO 216 size. It is one step in the A-series, which begins with A0 at 841 x 1189 mm — an area of exactly one square meter — and halves at each step.

What is A in the NATO phonetic alphabet?

The code word for A is “Alfa” (also spelled Alpha). It is spelled with an f rather than ph so speakers of any language pronounce it correctly. The alphabet was standardized by the ICAO in 1956.

What does A mean as a blood type?

Blood group A is one of the four main ABO groups, marked by A antigens on the red blood cells. With the Rh factor it becomes A positive (A+) or A negative (A−), depending on whether the Rh(D) antigen is present.

Why are there so many meanings for A?

A is the first letter of the alphabet, so it does double duty as a mark of rank and order — the top grade, the first item, the highest rating — on top of standing in for countless words and names. That is why the meanings below range from an A-list celebrity to A1 condition to the ampere.