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).