a

indefinite article, form of an used before consonants, mid-12c., a weakened form of Old English an "one" (see an). After c. 1600 the -n- also began to vanish before words beginning with a sounded -h-; 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.

It also is used before nouns of singular number and a few plural nouns when few or great many is interposed.

as in twice a day, etc., a reduced form of Old English an "on" (see on (prep.)), in this case "on each." The sense was extended from time to measure, price, place, etc.