What words are indefinite articles?
"a" and "an"
The only indefinite articles in English are "a" and "an".
We normally use "a" before a word that begins with a consonant sound and "an" before a word that begins with a vowel sound.
Historically these words come from an old form of the word "one" and that's essentially what they mean. So "a house" means "one house" without indicating or specifying which house is meant.