a (Noun) — A metric unit of length equal to one ten billionth of a meter (or 0.0001 micron); used to specify wavelengths of electromagnetic radiation.
a (Noun) — The 1st letter of the Roman alphabet.
a (Noun) — One of the four nucleotides used in building DNA; all four nucleotides have a common phosphate group and a sugar (ribose).
a (Noun) — (biochemistry) purine base found in DNA and RNA; pairs with thymine in DNA and with uracil in RNA.
a (Noun) — Any of several fat-soluble vitamins essential for normal vision; prevents night blindness, inflammation or dryness of the eyes.
a (Noun) — The blood group whose red cells carry the A antigen.
a (Noun) — The basic unit of electric current adopted under the Systeme International d'Unites. ex. "a typical household circuit carries 15 to 50 As"
a (Other) — Indefinite article, "an" is used before a vowel sound, "a" otherwise.