Determiner, also called determinative (abbreviated DET), is a term used in some models of grammatical description to describe a word or affix belonging to a class of noun modifiers. A determiner combines with a noun to express its reference. Examples in English include articles (the and a/an), demonstratives (this, that), possessive determiners (my, their), and quantifiers (many, both). Not all languages have determiners, and not all systems of grammatical description recognize them as a distinct category.

The linguistics term "determiner" was coined by Leonard Bloomfield in 1933. Bloomfield observed that in English, nouns often require a qualifying word such as an article or adjective. He proposed that such words belong to a distinct class which he called "determiners".

If a language is said to have determiners, any articles are normally included in the class. Other types of words often regarded as belonging to the determiner class include demonstratives and possessives. Some linguists extend the term to include other words in the noun phrase such as adjectives and pronouns, or even modifiers in other parts of the sentence.

Qualifying a lexical item as a determiner may depend on a given language's rules of syntax. In English, for example, the words my, your etc. are used without articles and so can be regarded as possessive determiners whereas their Italian equivalents mio etc. are used together with articles and so may be better classed as adjectives. Not all languages can be said to have a lexically distinct class of determiners.

In some languages, the role of certain determiners can be played by affixes (prefixes or suffixes) attached to a noun or by other types of inflection. For example, definite articles are represented by suffixes in Romanian, Bulgarian, Macedonian, and Swedish. In Swedish, bok ("book"), when definite, becomes boken ("the book"), while the Romanian caiet ("notebook") similarly becomes caietul ("the notebook"). Some languages, such as Finnish, have possessive affixes which play the role of possessive determiners like my and his.

Determiners may be predeterminers, central determiners or postdeterminers, based on the order in which they can occur.^{[citation needed]} For example, "all my many very young children" uses one of each. "My all many very young children" is not grammatical because a central determiner cannot precede a predeterminer.

Determiners are distinguished from pronouns by the presence of nouns.

Plural personal pronouns can act as determiners in certain constructions.