In grammatical theory, definiteness is a feature of noun phrases, distinguishing between entities which are specific and identifiable in a given context (definite noun phrases) and entities which are not (indefinite noun phrases).
There is considerable variation in the expression of definiteness across languages: some languages use a definite article (which can be a free form, a phrasal clitic, or an affix on the noun) to mark a definite noun phrase. Examples are:
Phrasal clitic: as in Basque: Cf. ("woman"), (woman-ART: "the woman"), (woman beautiful-ART: "the beautiful woman")
Noun affix: as in Romanian: ("man"), (man-ART: "the man"); (man-ART good: "the good man")
Prefix on both noun and adjective: Arabic (al-kitāb al-kabīr) with two instances of al- (DEF-book-DEF-big, literally, "the book the big")
Distinct verbal forms: as in Hungarian: (read-1sg.pres.INDEF a book-ACC.sg: "I read a book") versus (read-1sg.pres.DEF the book-ACC.sg: "I read the book")