Determiners are words that introduce nouns and specify which noun is being referred to. They include:
* Articles: "a," "an," "the"
* Demonstratives: "this," "that," "these," "those"
* Possessives: "my," "your," "his," "her," "its," "our," "their"
* Quantifiers: "some," "any," "many," "few," "much," "little"
* Numbers: "one," "two," "three," etc.
So while "the" is not an adjective (which describes nouns), it is a determiner that functions as a specifier for nouns.