A (disambiguation)
A is the first letter of the Latin and English alphabet. A may also refer to: == Science and technology == acceleration (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 ångström (Å) a unit of length (equal to 1×10−10 metres) area (A) attenuation coefficient (a) rotational constant (A) specific surface area (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, an antigen and a corresponding blood type in the ABO blood group system in primates such as humans 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 === Chemistry === A, historical symbol for chemical element argon, currently Ar A value, a measure of substituent effects on the stereochemistry of cyclohexane activity (chemistry) (a) chemical affinity (A) mass number of a nuclide (A) relative atomic mass (Ar) === Crystallography === The center of a hexagonal face of the Brillouin zone of a hexagonal lattice, in physics one of the reciprocal lattice vectors (a*) unit cell length (a) === 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 === Electricity and magnetism === A battery, a vacuum tube filament type battery A battery (size), a standard battery size Anode, an electrode absorbance (A) Ampere (A), unit of electric current Bohr radius (a0) gain (electronics) (A) Hall coefficient (AH) magnetic vector potential (A) Hyperfine coupling constant (a or A) === 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 ( A {\displaystyle \mathbb {A} } ) (U+1D538 in Unicode) Universal quantifier in symbolic logic (symbol ∀ or ∀ {\displaystyle \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 === Thermodynamics === a, a measure for the attraction between particles in the Van der Waals equation Hamaker constant (A) Helmholtz free energy (A) pre-exponential factor (A) Richardson's constant (A) thermal diffusivity (a) == 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 === Film and TV === ==== Films ==== A (1965 film), a short film animated by Jan Lenica A (1998 Indian film), an Indian film directed by Upendra A (1998 Japanese film), a documentary film about a Buddhist sect called "A" A (2002 film), a British documentary film A (Ad Infinitum), a 2021 Indian 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 channels ==== CTV 2, a Canadian television system formerly known as A-Channel and A "A"-logo for the Spanish network Antena 3.