In linguistics, definiteness is a semantic feature of noun phrases (NPs), distinguishing between referents/entities that are identifiable in a given context (definite noun phrases) and entities which are not (indefinite noun phrases). In English, for example, definite noun phrases preclude asking "which one?"
There is considerable variation in the expression of definiteness across languages and some languages do not express it at all. For example, in English definiteness is usually marked by the selection of determiner. Certain determiners, such as a/an, many, any, either, and some typically mark an NP as indefinite. Others, including the, this, every, and both mark the NP as definite. In some other languages, the marker is a clitic that attaches phonologically to the noun (and often to modifying adjectives), e.g. the Hebrew definite article ha- or the Arabic definite article al-. In yet other languages, definiteness is indicated by affixes on the noun or on modifying adjectives, much like the expression of grammatical number and grammatical case. In these languages, the inflections indicating definiteness may be quite complex. In the Germanic languages and Balto-Slavic languages, for example (as still in modern German and Lithuanian), there are two paradigms for adjectives, one used in definite noun phrases and the other used in indefinite noun phrases. In some languages, e.g. Hungarian, definiteness is marked on the verb.
Germanic, Romance, Celtic, Semitic, and auxiliary languages generally have a definite article, sometimes used as a postposition. Many other languages do not. Some examples are Chinese, Japanese, Finnish, and modern Slavic languages except Bulgarian and Macedonian. When necessary, languages of this kind may indicate definiteness by other means such as Demonstratives.
It is common for definiteness to interact with the marking of case in certain syntactic contexts. In many languages direct objects (DOs) receive distinctive marking only if they are definite. For example, in Turkish, the DO in the sentence adamları gördüm (meaning "I saw the men") is marked with the suffix -ı (indicating definiteness). The absence of the suffix means that the DO is indefinite ("I saw men").
In Serbo-Croatian (and in the Baltic languages Latvian and Lithuanian), and to a lesser extent in Slovene, definiteness can be expressed morphologically on prenominal adjectives. The short form of the adjective is interpreted as indefinite, while the long form is definite and/or specific:
In Japanese, a language that indicates noun functions with postpositions, the topic marker (wa) may include definiteness. For example, 馬は (uma wa) can mean "the horse", while 馬が (uma ga) can mean "a horse".
In some languages, the definiteness of the object affects the transitivity of the verb. In the absence of peculiar specificity marking, it also tends to affect the telicity of mono-occasional predications.
In some languages definiteness can be seen a morphological category of nouns. For example, in some Scandinavian languages, such as Swedish, definite nouns inflect with a dedicated set of suffixes. This is known in Swedish as the grammatical category of Species.
In Semitic languages the category of state is sometimes tied to definiteness, as some Semitic languages are said to distinguish between three morphological states: Indefinite (Absolute) State, Definite (Emphatic) State, and Construct State. Such a system exists for instance in Old Aramaic. Yet in other Semitic languages, like Hebrew or Arabic, definiteness is marked by a pro-clitic, and the state category relates only to the question whether a nominal is necessarily modified by a complement (and is thus in the construct state) or not (being in the free state).