adj. (also an before a vowel) (called the indefinite article) 1 (as an unemphatic substitute) one, some, any. 2 one like (a Judas). 3 one single (not a thing in sight). 4 the same (all of a size). 5 in, to, or for each (twice a year; $20 a man; seven a side).
[ weakening of OE an one; sense 5 orig. = A(2) ]