Indefinite article

the status of a noun phrase that either (i) refers to a specific entity (or set of entities) known to the speaker, but assumed not to be familiar to, or identifiable by, the addressee(s) (e.g., a tiger in we saw a tiger today) or (ii) denotes a non-specific entity (or set of entities), whose identity is unknown or unknowable to the speaker (e.g., a tiger in I’ve never seen a tiger)