In English grammar, "a", and its variant "an", is an indefinite article, used to introduce noun phrases.
ɪn ˈɪŋɡlɪʃ ɡɹˈæmɚ, "ˈeɪ", ænd ɪts vˈɛɹiənt "æn", ɪz ɐn ɪndˈɛfɪnət ˈɑːɹɾɪkəl, jˈuːzd tʊ ˌɪntɹədˈuːs nˈaʊn fɹˈeɪzᵻz.