A1
used before a noun to refer to a single thing or person but not a particular thing or person or not one that you have referred to before
Dictionary examples:
A cat ate my homework. Really! (0.0)
I need a nap and a cookie. (0.0)
I need a vacation from my vacation. (0.0)
She's got a boyfriend. (9.5)
Is he a friend of yours? (12.3)
Someone left an umbrella here yesterday. (14.0)