Science

• A battery

• 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

• A+ and A-, human blood types

• Adenine, nucleic acid

• Alanine, amino acid

• Adenosine, nucleoside

• ATC code A Alimentary tract and metabolism, a section of the Anatomical Therapeutic Chemical Classification System

• The anode, a component of a vacuum tube

• The center of a hexagonal face of the Brillouin zone of a hexagonal lattice, in physics

Astronomy

• A whitish-blue class of stars in the Morgan-Keenan system

• A January 1 through 15 discovery, in the provisional designation of a comet (for example, C/1760 A1, the Great Comet of 1760) or asteroid

• As a comet designation prefix, indicates an object that was at first mistakenly identified as a comet, but is actually a minor planet (for example, )

• The semi-major axis of an orbit

Abbreviations for physical and chemical quantities and units

• acceleration (a)

• the unique, or one of the, reciprocal lattice vector(s) (a*)

• attenuation coefficient (a)

• activity (chemistry) (a)

• specific surface area (a)

• thermal diffusivity (a)

• unit cell length (a)

• Bohr radius (a₀)

• Adsorption (a)

• annum (a), for year

• are (a), a unit of area equal to 100 square metres

• atto- (a), the SI prefix meaning 10⁻¹⁸

• a, a measure for the attraction between particles in the Van der Waals equation

• magnetic vector potential (A')

• absorbance (A)

• area (A)

• A value (A), a measure of substituent effects on the stereochemistry of cyclohexane

• Helmholtz free energy (A)

• mass number (A)

• pre-exponential factor (A)

• rotational constant (A)

• Hall coefficient (A_H)

• relative atomic mass (A_r)

• ampere (A), the SI base unit of electric current

• ångström (Å), a unit of length equal to 1 metres

• chemical affinity (A)

• gain (A)

• Hamaker constant (A)

• hyperfine coupling constant (a or A), see Electron paramagnetic resonance#Hyperfine coupling

• Richardson's constant (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 ($\backslash mathbb\{A\}$ (U+1D538 in Unicode)

• The universal quantifier in symbolic logic (symbol ∀ or $\backslash forall$ (an inverted letter A)

• Mills' constant is represented by the symbol A

• Glaisher–Kinkelin constant is also often represented by the symbol A

Computing

• A*, a pathfinding algorithm

• A-0 System, early computer compiler

• A+ (programming language)

• A Sharp (Axiom), properly known as "A#"

• BASIC A+

• , the HTML element for an anchor tag

• A Sharp (.NET) (A port of Ada to the Microsoft .NET Platform)

• The most basic of the three levels of web site accessibility measured by WCAG

The arts

• A (musical note), the sixth musical note

• A (Jethro Tull album), 1980 album by progressive rock band Jethro Tull

• A (band) is a rock band from Leeds, formed in 1995

• A (Cass McCombs album), 2003 album by Cass McCombs

• "A" (song), Ayumi Hamasaki's 10th single, released 1999

• A (TV system), a Canadian television system formerly known as A-Channel

• "A", song by Cartel (band)

• "A", song by the band Barenaked Ladies

• A, or "side A," refers to the top or first side of a vinyl record

• "A", major poem by 20th century author Louis Zukofsky

• a, A Novel, by 20th century artist Andy Warhol

Cinema

• A (Kannada film) is a Kannada movie directed by Upendra

• A (film) is a 1965 short animated film by Jan Lenica

• A, a rating in India's Central Board of Film Certification

• A certificate a film rating in Canada

Linguistics

• A and an, the indefinite article in English

• A (kana), the Japanese kana あ and ア which may be used to represent Asia

• A (Cyrillic), the first letter of the Cyrillic alphabet

• a, the IPA representation of the open front unrounded vowel

• ɑ, the IPA representation of the open back unrounded vowel

Sports

• Class A, a classification of Minor league baseball, commonly referred to as "Single A"

Miscellanea

• A-Frame, a type of structure

• A (New York City Subway service), also known as the "A Eighth Avenue Express"

• A (S-train) a service on the S-train network in Copenhagen

• Alton Railroad (reporting mark)

• A, the alternate name for Brfxxccxxmnpcccclllmmnprxvclmnckssqlbb11116, a Swedish child born in 1991

• "The A's", nickname for Oakland Athletics

• A, the U.S. ticker symbol for Agilent Technologies

• A within a circle, used in anarchist symbolism

• An ace in a deck of playing cards

• In Christian theology, alpha is a metaphor for the beginning/creation of time and matter. It is sometimes translated to A

• A, a song in Dance Dance Revolution Extreme

• The A series of paper sizes defined by ISO 216, the most frequently used being the A4 size.

• A (grade), an academic grade in North America

See also

• Å (disambiguation)

• A band (disambiguation)

• A class (disambiguation)

• Class A (disambiguation)

• Alpha (disambiguation)

• Aleph (disambiguation)