In English grammar, a determiner is a word or a group of words that specifies, identifies, or quantifies the noun or noun phrase that follows it. Also known as a prenominal modifier.
Determiners include articles (a, an, the); cardinal numbers (one, two, three . . .) and ordinal numbers (first, second, third . . .); demonstratives (this, that, these, those); partitives (some of, piece of, and others); quantifiers (most, all, and others); and possessive determiners (my, your, his, her, its, our, their.)"The determiner class is one of the structure classes that straddle the line between a word class and a function. On the one hand, our most common determiners, the articles, do indeed constitute a small, closed structure class. At the other end of the spectrum are the possessive nouns, which function as determiners while retaining their membership in the open class 'noun.' In between are the subclasses of determiners that belong to the closed pronoun class: Demonstrative, possessive, and indefinite pronouns all function as determiners; and, of course, as pronouns they also function as nominals (in fact, 'pronominal' would be a more accurate label than 'pronoun').
"Determiners signal nouns in a variety of ways: They may define the relationship of the noun to the speaker or listener (or reader); they may identify the noun as specific or general; they may quantify it specifically or refer to quantity in general."
(Martha Kolln and Robert Funk, Understanding English Grammar, 5th ed. Allyn and Bacon, 1998. Digital FAQs