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).
For examples of determiners and use, see the box on the use in English, English determiners
Most determiners have been traditionally classed along with either adjectives or pronouns, and this still occurs in classical grammars: 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. These classical interpretations of determiners map to some of the linguistic properties related to determiners in modern syntax theories, such as deictic information, definiteness and genitivity. 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.
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.
This article or section appears to contradict itself on the status of numerals as a determiner.(May 2018)
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.
An article is a word that is used with a noun (as a standalone word or a prefix or suffix) to specify grammatical definiteness of the noun, and in some languages extending to volume or numerical scope.Some example of articles are a, an, the.
Demonstratives are words, such as this and that, used to indicate which entities are being referred to and to distinguish those entities from others. They are usually deictic, which means their meaning changes with context.
Possessive determiners modify a noun by attributing possession (or other sense of belonging) to someone or something, and usually reflect the noun's genitive case. They are also known as possessive adjectives.
Quantifiers indicate quantity. Some examples of quantifiers include: all, some, many, few, and no. Quantifiers are also dependent of a noun. It is important to acknowledge that quantifiers only indicate a vague quantity of objects, not a specific number, such as twelve, dozen, first, single, or once, which would be considered numerals. 
Distributive determiners, also called distributive adjectives, consider members of a group separately, rather than collectively. Words such as each, any, either, and neither are examples of distributive determiners. This type of determiner also depends on a noun. These determiners are not to be confused with distributive pronouns, which can operate without a noun.
Interrogatives are used to ask a question, such as which, what, and whose (personal possessive determiner). These determiners also depend on a noun.
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. Under this assumption, every noun in a syntax tree is dominated by a determiner. There are many examples in natural language where nouns appear without a determiner, yet in determiner phrase grammars there must still be a determiner. To account for this, syntacticians consider the head of the determiner phrase to be an unpronounced null determiner. These grammar theories are either based on X-bar theory or descend from it, which requires that every noun has a corresponding determiner (or specifier). In the cases 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, an X-bar specific form of the null determiner. Noun phrases that contain only a noun and do not have a determiner present are known as bare noun phrases. For more detail on theoretical approaches to the status of determiners, see Noun phrase § Noun phrases with and without determiners.
Under the Universal Grammar theory, most facets of language are inherent, and only idiosyncrasies of languages are learned. Notice that determiners and subsequently their phrases would have to inherently be part of Universal Grammar in order for Determiner Phrase theory and Universal Grammar theory to coexist.
Some theoreticians unify determiners and pronouns into a single class. See Pronoun: Theoretical considerations. This is consistent with the determiner phrase viewpoint, whereby a determiner, rather than the noun that follows it, is taken to be the head of the phrase.