I. determiner 1. used 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 you.
Compare with the.
2. used with units of measurement to mean one such unit • a hundred
• a quarter of an hour.
3. [with negative] — one single; any • I simply haven’t a thing to wear.
4. used when mentioning the name of someone not known to the speaker • a Mr Smith telephoned.
5. someone like (the name specified) • you’re no better than a Hitler.
6. used to indicate membership of a class of people or things • he is a lawyer
• this car is a BMW.
7. in, 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’. / usage: On the question of using a or an before words beginning with h, see usage at an.