# A (disambiguation)

A is the first letter of the Latin and English alphabet.
A may also refer to:

## Science and technology
### Quantities and units

* a, a measure for the attraction between particles in the Van der Waals equation
* A value, a measure of substituent effects on the stereochemistry of cyclohexane
* absorbance (A)
* acceleration (a)
* activity (chemistry) (a)
* adsorption (a)
* annum (a), for year
* are (a), a unit of area (equal to 100 square metres; redirects to hectare)
* atto- (a-), the SI prefix meaning 10$^{−18}$
* Ampere (A), unit of electric current
* ångström (Å) a unit of length (equal to 1×10$^{−10}$ metres)
* area (A)
* attenuation coefficient (a)
* Bohr radius (a$_0$)
* chemical affinity (A)
* gain (electronics) (A)
* Hall coefficient (A$_H$)
* Hamaker constant (A)
* Helmholtz free energy (A)
* Hyperfine coupling constant (a or A)
* magnetic vector potential (A)
* mass number of a nuclide (A)
* pre-exponential factor (A)
* one of the reciprocal lattice vectors (a*)
* relative atomic mass (A$_r$)
* Richardson's constant (A)
* rotational constant (A)
* specific surface area (a)
* thermal diffusivity (a)
* unit cell length (a)

### Astronomy

* A whitish-blue class of stars in the Morgan–Keenan system; see Stellar classification
* Part of the provisional designation of a comet (e.g., C/1760 A1), indicating a January 1 through 15 discovery
* The semi-major axis of an orbit

### Biology

* A+ and A−, human blood types
* Vitamin A, also called retinol, an essential human nutrient
* Haplogroup A (mtDNA), a human mitochondrial DNA (mtDNA) haplogroup
* Haplogroup A (Y-DNA), a Y-chromosomal DNA (Y-DNA) haplogroup
* Adenine, a nucleic acid
* Adenosine, a nucleoside
* A (or Ala), abbreviation for amino acid alanine
* ATC code A, Alimentary tract and metabolism, a section of the Anatomical Therapeutic Chemical Classification System

### Computing
#### Programming languages

* A+ (programming language)
* A$^♯$, an object-oriented functional programming language
* A Sharp (.NET) (also written "A#"), a port of Ada to the Microsoft .NET Platform
* BASIC A+

#### Other uses in computing

* A* search algorithm, a pathfinding algorithm
* A-0 System, an early computer compiler
* <a></a>, the HTML element for an anchor tag
* a, equivalent (qname) to the RDF Schema Property rdf:type
* A level of web accessibility defined by the Web Content Accessibility Guidelines
* The address record type of DNS, defined in RFC 1035

### Mathematics and logic

* A numerical digit meaning ten in hexadecimal and other positional numeral systems with a radix of 11 or greater
* Sometimes in blackboard bold represents the algebraic numbers ($\mathbb {A}$) (U+1D538 in Unicode)
* Universal quantifier in symbolic logic (symbol ∀ or $\forall$, an inverted letter A)
* Universal affirmative, one of the four types of categorical proposition in logic
* Mills' constant is represented by the symbol A
* Glaisher–Kinkelin constant is also often represented by the symbol A

### Other uses in science and technology

* A battery, a vacuum tube filament type battery
* A battery (size), a standard battery size
* Anode, an electrode
* A, historical symbol for chemical element argon, currently Ar
* The center of a hexagonal face of the Brillouin zone of a hexagonal lattice, in physics

## Arts and media
### Books

* "A", a poem by twentieth century author Louis Zukofsky
* a, A Novel, by twentieth century artist Andy Warhol
* A, the name of the Fourth Raikage, the leader of Kumogakure in the Naruto manga series
* A (Pretty Little Liars), the main antagonist in the Pretty Little Liars book and TV series
* A (for "adultery"), the titular letter in the novel The Scarlet Letter
* "A" Is for Alibi, the first novel in Sue Grafton's "Alphabet mystery" series, published in 1982
* Matthew Arnold, who used the pseudonym "A"
* Isaac Asimov, who used the pseudonym "Dr. A"
* Alexander Pope, who used the pseudonym "A"

### Film and TV
#### Films

* A (1965 film), a short film animated by Jan Lenica
* A (1998 Kannada film), an Indian movie directed by Upendra
* A (1998 Japanese film), a documentary film about a Buddhist sect called "A"
* A (2002 film), a British documentary film

#### Film ratings

* A, a rating in India's Central Board of Film Certification
* A certificate, a film rating used by the Canadian motion picture rating system
* A, a British Board of Film Classification rating used from 1912 to 1982, see History of British film certificates

#### Television

* CTV 2, a Canadian television system formerly known as A-Channel and A
* A, the main antagonist and unknown character of the show Pretty Little Liars
* "A" (The Walking Dead), an episode of the television series The Walking Dead
* A, code name of Ange, the main protagonist of the anime show Princess Principal
* "A"-logo for the Spanish network Antena 3.
* A, the production code for the 1963 Doctor Who serial An Unearthly Child

### Music

* A (musical note), the sixth solfège note
* A major, a scale
* A minor, a scale
* A major chord; see Chord names and symbols (popular music)

#### Performers

* A (band), a British alternative rock band

#### Albums

* A (Agnetha Fältskog album), 2013
* A (Cass McCombs album), 2003
* A (Jimmy Raney album), 1957
* A (Jethro Tull album), 1980
* A (Usher and Zaytoven album), 2018
* A, a Denki Groove album (read as "Ace")
* A (Ayumi Hamasaki EP), 1999
* A (single album), a 2015 single by the Korean band Big Bang
* A! (album), by Alexa Feser, 2019
* A' (album), by Afrirampo, 2004

#### Songs

* A or Side A, the top or first side of a gramophone record
* "A" (Rainbow song), a 2010 single by Rainbow
* "A", song by A from 'A' vs. Monkey Kong
* "A", song by Got7 from Got Love (2014)
* "A", song by the band Barenaked Ladies from Maybe You Should Drive
* "A", song by Cartel from Chroma
* "A", a song in the Dance Dance Revolution video game series, by DJ Amuro

### Other media

* /a/, the anime and manga section on 4chan
* A, a Japanese video game rating in the Computer Entertainment Rating Organization suitable for all ages.

## Geography

* Austria, on the vehicle registration plates of the European Union
* "The A", a nickname for the U.S. city of Atlanta, Georgia

## Linguistics
### Letters

* A (or a), the first letter of the Latin alphabet
* A (Indic), the many form of brahmic "a"
* A (Bengali), the romanized name of the Bengali letter অ
* A (cuneiform), the romanized name of a cuneiform sign
* A (Cyrillic) (A or а), the first letter of the Cyrillic alphabet
* A (kana) (or a), the romanized name of the Hiragana syllable letter あ and of the Katakana syllable letter ア used in Japanese, which may also be used to represent Asia
* Alpha (Α or α), the first letter of the Greek alphabet

### Other uses in linguistics

* A and an, the indefinite article in English
* A, a glossing abbreviation for agent-like argument of canonical transitive verb
* a, the IPA representation of the open front unrounded vowel
* ɑ, the IPA representation of the open back unrounded vowel
* a., an abbreviation of ante (Latin for "before") used in etymologies

## People

* A (pharaoh), an Egyptian pharaoh
* A Martinez, an American actor and singer
* Matthew Arnold, who used the pseudonym "A"
* Isaac Asimov, who used the pseudonym "Dr. A"
* Alexander Pope, who used the pseudonym "A"
* Brfxxccxxmnpcccclllmmnprxvclmnckssqlbb11116, a Swedish child born in 1991, alternately called "A"

## Philosophy and religion

* A, the Dominical letter of a common year starting on Sunday
* Scarlet A, symbol for atheism adopted by the Out Campaign, a public awareness initiative proposed by Elizabeth Cornwell and Richard Dawkins
* Alpha, (sometimes A) in Christian theology, a metaphor for the beginning/creation of time and matter
* An anarchist symbol, an A within a circle
* The Gregory-Aland siglum for the Codex Alexandrinus.

## Transport
### Automobile

* Ford Model A, a name used by two separate cars produced by the Ford Motor Company:
** Ford Model A (1903–04)
** Ford Model A (1927–31)
* Geometry A, an electric mid-size sedan by Geely Auto

### Rail

* A (S-train) a service on the S-train network in Copenhagen
* Line A of the Buenos Aires Subte
* Line A (Prague Metro)
* Mexico City Metro Line A
* NZR A class (disambiguation) two New Zealand steam locomotive classes
* RER A, a line in the RER of Paris
* Lyon Metro Line A
* Toei Asakusa Line, a subway service operated by the Tokyo Metropolitan Bureau of Transportation (Toei), labeled
* The official West Japan Railway Company service symbol for:
** Akō Line, Biwako Line, Kobe Line, Kyoto Line, Hokuriku Main Line, San'yō Main Line, and JR West Tōkaidō Line.
** San'in Main Line.
* United States
** A (Los Angeles Railway)
** A (New York City Subway service), also known as the "A Eighth Avenue Express"
** Alton Railroad (reporting mark)
** A Line (Los Angeles Metro)
* Britain
** LCDR A class, British 0-4-4T steam locomotives introduced in 1875
** LD&ECR Class A, British 0-6-2T steam locomotives introduced in 1895
** LNWR Class A, British 0-8-0 steam locomotives introduced in 1893
** NER Class A, British 2-4-2T steam locomotives introduced in 1886
** SER A class, British 4-4-0 steam locomotives introduced in 1879
** Taff Vale Railway A class, British 0-6-2T steam locomotives introduced in 1914

### Ships

* A (motor yacht) (M/Y A), a 119m yacht owned by Andrey Melnichenko
* A (sailing yacht) (S/Y A), a 143m yacht owned by Andrey Melnichenko
* SMS A, a Kaiserliche Marine tanker

## Other uses

* A (grade), an academic grade
* A series of paper sizes defined by ISO 216, e.g. A4, A5 sizes
* A formation, an American football offensive formation
* A-frame, a type of structure
* Aalto University, public research university in Espoo, Finland
* Ace, a playing card
* Agilent Technologies, with U.S. ticker symbol A
* Sentinel Peak (Arizona), more commonly known as "A" Mountain for the human-made rock formation in the shape of the letter
* Alfa, the military time zone code for UTC+01:00
* Farmall A, a tractor produced by International Harvester from 1939 to 1947
* Oakland A's, or the Athletics, a professional baseball team based in Oakland, California, United States