In linguistics, definiteness is a semantic feature of noun phrases (NPs), distinguishing between referents or senses that are identifiable in a given context (definite noun phrases) and entities which are not (indefinite noun phrases). The prototypical definite NP picks out a unique, familiar, specific referent.[1]

There is considerable variation in the expression of definiteness across languages, and some languages such as Japanese do not generally mark it.[2] In others, such as English, it is usually marked by the selection of determiner.[3] In still other languages, such as Danish, definiteness is marked morphologically.[4]

The theoretical distinction between grammatical definiteness and cognitive identifiability has the advantage of enabling us to distinguish between a discrete (grammatical) and a non-discrete (cognitive) category.[5][p. 84][a]

In English, definiteness is usually marked by the selection of determiner. Certain determiners, such as a, an, many, and some, along with numbers (e.g., four items), typically mark an NP as indefinite. Others, including the, that, and genitive NPs (e.g., my brother) typically mark the NP as definite.[3]

A number of tests have been proposed to distinguish definite from indefinite NPs. "Each has a foundation in intuition, as well as some degree of grammatical effect. However, it is not clear that any of them corresponds cleanly to formal categories."[1]

Germanic, Romance, Celtic, Semitic, and auxiliary languages generally have a definite article, often preposed but in some cases postposed. 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.[7]

It is common for definiteness to interact with the marking of case in certain syntactic contexts. In many languages, a direct object receives distinctive marking only if it is definite. For example, in Turkish, the direct object in the sentence adamı gördüm (meaning "I saw the man") is marked with the suffix (indicating definiteness)[7] (p. 204). The absence of the suffix on a direct object in Turkish means that it is indefinite and, in the absence of the indefinite article bir, no longer explicitly singular: adam gördüm ("I saw a man/I saw men"), .

In Serbo-Croatian, in the Baltic languages Latvian and Lithuanian, and, to a lesser extent in Slovene, definiteness can be expressed morphologically on prenominal adjectives.[8] The short form of the adjective is interpreted as indefinite, while the long form is definite or specific:

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 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.[citation needed]