A determiner, also called determinative (abbreviated DET), is a word, phrase, or affix that occurs together with a noun or noun phrase and serves to express the reference of that noun or noun phrase in the context. That is, a determiner may indicate whether the noun is referring to a definite or indefinite element of a class, to a closer or more distant element, to an element belonging to a specified person or thing, to a particular number or quantity, etc. Common kinds of determiners include definite and indefinite articles (like the English the and a or an), demonstratives (this and that), possessive determiners (my and their), quantifiers (many, few and several), numerals, distributive determiners (each, any), and interrogative determiners (which).
Most determiners have been traditionally classed either along with adjectives or with pronouns, and this still occurs: for example, demonstrative and possessive determiners are sometimes described as demonstrative adjectives and possessive adjectives or as (adjectival) demonstrative pronouns and (adjectival) possessive pronouns respectively. However, modern theorists of grammar prefer to distinguish determiners as a separate word class from adjectives, which are simple modifiers of nouns, expressing attributes of the thing referred to. This distinction applies particularly in languages like English that use definite and indefinite articles, frequently as a necessary component of noun phrases - the determiners may then be taken to be a class of words that includes the articles as well as other words that function in the place of articles. (The composition of this class may depend on the particular language's rules of syntax; for example, in English the possessives my, your etc. are used without articles and so can be regarded as 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. (For example, 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.
X-bar theory contends that every noun has a corresponding determiner (or specifier). In a case where a noun does not have an explicit determiner (as in physics uses mathematics), X-bar theory hypothesizes the presence of a zero article, or zero determiner. Noun phrases that contain only a noun and do not have a determiner present are known as bare noun phrases.
Some modern grammatical approaches regard determiners (rather than nouns) as the head of their phrase and thus refer to such phrases as determiner phrases rather than noun phrases. For more detail on theoretical approaches to the status of determiners, see Noun phrase § Noun phrases with and without determiners.
Universal Grammar is the theory that all humans are born equipped with grammar, and all languages share certain properties. There are arguments that determiners are not a part of Universal Grammar and are instead part of an emergent syntactic category. This has been shown through the studies of some languages' histories, including Dutch.
Determiners may be subcategorized as predeterminers, central determiners and postdeterminers, based on the order in which they can occur. For example, "all my seventeen very young children" uses one of each. "My all seventeen very young children" is ungrammatical because a central determiner cannot precede a predeterminer.