1.noun(grammar) A member of a class of words functioning in a noun phrase to identify or distinguish a referent without describing or modifying it.
2.noun(grammar) A dependent function in a noun phrase marking the NP as definite or indefinite. This function is usually filled by words in the determinative class but may be filled by other elements such as a genitive pronoun.
3.nounSomething that determines, or helps someone to determine, something else.