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 (a₀)
• 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

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 (${\displaystyle \mathbb {A} }$) (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

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

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

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)

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.

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
• 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

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