Definition of

A

(noun, body) the blood group whose red cells carry the A antigen
(noun, communication) the 1st letter of the Roman alphabet
(noun, quantity) the basic unit of electric current adopted under the Systeme International d'Unites
(noun, quantity) a metric unit of length equal to one ten billionth of a meter (or 0.0001 micron); used to specify wavelengths of electromagnetic radiation
(noun, substance) (biochemistry) purine base found in DNA and RNA; pairs with thymine in DNA and with uracil in RNA
(noun, substance) one of the four nucleotides used in building DNA; all four nucleotides have a common phosphate group and a sugar (ribose)
(noun, substance) any of several fat-soluble vitamins essential for normal vision; prevents night blindness or inflammation or dryness of the eyes

via WordNet, Princeton University

Synonyms of A

Hyponyms: dehydroretinol, retinol, vitamin a1, vitamin a2

Hypernyms: alphabetic character, base, blood group, blood type, current unit, fat-soluble vitamin, letter, letter of the alphabet, metric linear unit, nucleotide, purine

indefinite article, c.1150, a variation of O.E. an (see an) in which the -n- began to disappear before consonants, a process mostly complete by mid-14c. The -n- also was retained before words beginning with a sounded -h- until c.1600; it still is retained by many writers before unaccented syllables in h- or (e)u-, but is now no longer normally spoken as such. The -n- also lingered (especially in southern England dialect) before -w- and -y- through 15c.
as in twice a day, etc., from O.E. an "on," in this case "on each." The sense was extended from time to measure, price, place, etc. The habit of tacking a onto a gerund (as in a-hunting we will go) died out 18c.

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