a1

 
Pronunciation: /ə, eɪ/
(an before a vowel sound)

determiner

  • 1used when mentioning someone or something for the first time in a text or conversation: a man came out of the room it has been an honour to meet youCompare with the.
  • used with units of measurement to mean one such unit: a hundred a quarter of an hour
  • [with negative] one single; any: I simply haven’t a thing to wear
  • used when mentioning the name of someone not known to the speaker: a Mr Smith telephoned
  • someone like (the name specified): you’re no better than a Hitler
  • 2used to indicate membership of a class of people or things: he is a lawyer this car is a BMW
  • 3in, to, or for each; per (used when expressing rates or ratios): typing 60 words a minute a move to raise petrol prices by 3p a litre

Origin:

Middle English: weak form of Old English ān 'one'

On the question of using a or an before words beginning with h, see an (usage).