A (disambiguation)

A is the first letter of the Latin and English alphabet, but its significance goes beyond being a mere letter. In this comprehensive article, we explore the various meanings and references associated with the letter A.

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): a measure of the amount of light absorbed by a substance.
acceleration (a): the rate of change of velocity.
activity (chemistry) (a): a measure of the effective concentration of a solute in a solution.
adsorption (a): the process of molecules adhering to the surface of a solid.
annum (a): a unit of time equal to one year.
are (a): a unit of area equal to 100 square meters.
atto- (a-): the SI prefix representing 10^-18.
Ampere (A): a unit of electric current.
ångström (Å): a unit of length equal to 1×10^-10 meters.
area (A): the extent or measurement of a surface.
attenuation coefficient (a): a measure of how a material attenuates or weakens a beam of energy.
Bohr radius (a0): a physical constant that represents the size of the hydrogen atom.
chemical affinity (A): the attraction or force between atoms or molecules.
gain (electronics) (A): the amplification factor of an electronic device.
Hall coefficient (AH): a parameter that characterizes the behavior of electric currents in solid-state materials.
Hamaker constant (A): a parameter used to describe the attractive forces between particles or surfaces.
Helmholtz free energy (A): a thermodynamic potential that describes the work that can be extracted from a system at a constant temperature and volume.
Hyperfine coupling constant (a or A): a parameter that characterizes the interaction between the magnetic moments of electrons and nuclei.
magnetic vector potential (A): a vector field that represents the magnetic potential energy of a magnetic field.
mass number of a nuclide (A): the total number of protons and neutrons in an atomic nucleus.
pre-exponential factor (A): a coefficient used in the Arrhenius equation to describe the temperature dependence of reaction rates.
*reciprocal lattice vectors (a)**: vectors that describe the reciprocal lattice of a crystal.
relative atomic mass (Ar): the average mass of an element's isotopes relative to the mass of carbon-12.
Richardson's constant (A): a fundamental physical constant that relates the current density to the temperature and work function of a metal.
rotational constant (A): a parameter that characterizes the rotational energy levels of a molecule.
specific surface area (a): the surface area per unit mass of a material.
thermal diffusivity (a): a measure of how quickly heat diffuses through a material.
unit cell length (a): the length of a unit cell in a crystal lattice.

Astronomy

A: a whitish-blue class of stars in the Morgan-Keenan system, representing one of the stellar classifications.
A: part of the provisional designation of a comet, indicating a January 1 through 15 discovery.
A: the semi-major axis of an orbit.

Biology

A+ and A-: human blood types.
Vitamin A: also known as retinol, an essential human nutrient.
Haplogroup A (mtDNA): a human mitochondrial DNA haplogroup.
Haplogroup A (Y-DNA): a Y-chromosomal DNA haplogroup.
Adenine: a nucleic acid.
Adenosine: a nucleoside.
A (or Ala): an abbreviation for the 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: a numerical digit representing ten in hexadecimal and other positional numeral systems with a radix of 11 or greater.
A: sometimes represented in blackboard bold, it represents the algebraic numbers.
Universal quantifier: represented by the symbol ∀ or ∀, an inverted letter A in symbolic logic.
Universal affirmative: one of the four types of categorical proposition in logic.
Mills' constant: represented by the symbol A.
Glaisher-Kinkelin constant: 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 the chemical element argon.
The center of a hexagonal face of the Brillouin zone: 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 (The Scarlet Letter): the titular letter in the novel The Scarlet Letter.
"A Is for Alibi": the first novel in Sue Grafton's "Alphabet mystery" series.
Authors who used the pseudonym "A": Matthew Arnold, Isaac Asimov, and Alexander Pope.

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.

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: the 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 musical scale.
A minor: a musical scale.
A major chord: a chord commonly used in 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 (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": a song by A from 'A' vs. Monkey Kong.
"A": a song by Got7 from Got Love (2014).
"A": a song by the band Barenaked Ladies from Maybe You Should Drive.
"A": 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: represented on the vehicle registration plates of the European Union.
"The A": a nickname for the U.S. city of Atlanta, Georgia.

Linguistics

Letters

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): the first letter of the Cyrillic alphabet.
A (kana): the romanized name of the Hiragana syllable letter あ and 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 the agent-like argument of a 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.
Authors who used the pseudonym "A": Matthew Arnold, Isaac Asimov, and Alexander Pope.
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 by Elizabeth Cornwell and Richard Dawkins.
Alpha: sometimes represented as A in Christian theology, a metaphor for the beginning/creation of time and matter.
Anarchy symbol: an A within a circle, representing anarchism.

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 A.
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

M/Y A (motor yacht): a 119m yacht owned by Andrey Melnichenko.
S/Y A (sailing yacht): 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.
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.