Noun phrases (NPs) beginning with the or a/an are prototypical definite and indefinite NPs in English. The two main theories about the meaning of definiteness are uniqueness and familiarity. Both properties characterize most occurrences of definite descriptions although there are examples which defy one or the other or both theories. Existential sentences have become criterial for distinguishing indefinites from definites, and have led to broadening of both categories to include a variety of other NP forms. Information status approaches propose a hierarchy of NP types, rather than a simple binary distinction. The expression of definiteness varies from language to language.