English determiners (also known as determinatives) are words – such as the, a, each, some, which, this, and numerals such as six – that are most commonly used with nouns to specify their referents. The determiners form a closed lexical category in English.

The syntactic role characteristically performed by determiners is known as the determinative function (see § Terminology). A determinative combines with a noun (or, more formally, a nominal; see English nouns § Internal structure) to form a noun phrase (NP). This function typically comes before any modifiers in the NP (e.g., some very pretty wool sweaters, not *very pretty some wool sweaters). The determinative function is typically obligatory in a singular, countable, common noun phrase (compare I have a new cat to *I have new cat).

Semantically, determiners are usually definite or indefinite (e.g., the cat versus a cat), and they often agree with the number of the head noun (e.g., a new cat but not *many new cat). Morphologically, they are usually simple and do not inflect.

The most common of these are the definite and indefinite articles, the and a(n). Other determiners in English include the demonstratives this and that, and the quantifiers (e.g., many, and none) as well as the numerals. Determiners also occasionally function as modifiers in noun phrases (e.g., the many changes), determiner phrases (e.g., many more) or in adjective or adverb phrases (e.g., not that big). They may appear on their own without a noun, similar to pronouns (e.g., I'll have some), but they are distinct from pronouns.

Some sources, e.g. Cambridge Dictionary, Longman Dictionary, Collins Dictionary, and Collins COBUILD English grammar distinguish between predeterminers and determiners. Following this distinction, determiners can't be used directly next to each other (not: the my or my the). However, it is possible to put a predeterminer before a determiner (e.g. all the).

Words and phrases can be categorized by both their syntactic category and their syntactic function. In the clause the dog bit the man, for example, the dog belongs to the syntactic category of noun phrase and performs the syntactic function of subject. The distinction between category and function is at the heart of a terminological issue surrounding the word determiner: various grammars have used the word to describe a category, a function, or both.

Some sources, such as A Comprehensive Grammar of the English Language, use determiner as a term for a category as defined above and determinative for the function that determiners and possessives typically perform in a noun phrase (see § Functions). Others, such as The Cambridge Grammar of the English Language (CGEL), make the opposite terminological choice. And still others (e.g., The Grammar Book) use determiner for both the category and the function. This article uses determiner for the category and determinative for the function in the noun phrase.

The lexical category determiner is the class of words described in this article. They head determiner phrases, which can realize the functions determinative, predeterminative, and modifier: