A modifying word that precedes a noun to clarify which noun is being referred to (e.g., this, my, some).

There are five categories of determiners:

Articles: a, the
Demonstratives: this, that, etc.
Possessive adjectives/possessive determiners: my, our
Interrogative adjectives/interrogative determiners: whose, which
Quantifiers: some, a few, several
Numbers: Cardinal (one, two, three) and ordinal (first, second, third) numbers are also used as determiners.