A. Similis

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 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
* Alanine, an amino acid
* 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
* , 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 ($\mathbb$) (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 major 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!, Alexa Feser 2019
* A' (album), an album by Afrirampo

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 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 (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
* , the IPA representation of the open front unrounded vowel
* , 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 initiated 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

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)
* NZR A class (disambiguation) two New Zealand steam locomotive classes
* 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
* , 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
* Ace, a playing card
* Agilent Technologies, with U.S. ticker symbol A
* Sentinel Peak (Arizona), more commonly known as "A" Mountain for the man-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

See also

* Å (disambiguation)
* ⓐ and Ⓐ, see Enclosed Alphanumerics
* Class A (disambiguation)
* Model A (disambiguation)
* Circled a (disambiguation)
* Single-A, or Class A, a classification of minor league baseball
* A band (disambiguation)
* A class (disambiguation)
* A type (disambiguation)
* Aleph (disambiguation)
* Alpha (disambiguation)
* A1 (disambiguation)
* AA (disambiguation)
* AAA (disambiguation)
* AAAA (disambiguation)

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