One of a small group of words - a word class - that precedes and pre-modifies a noun and creates a noun phrase, e.g. a, the, some, this, that, those, each.
Determiners include the three 'articles' (i.e. a, an, the) and similar words: e.g. some, those, many, their. Each of these are said to determine the number or 'definiteness' of their noun, e.g. 'That man is the one!'
Confusingly, determiners can themselves be pre-modified by 'pre-determiners', e.g. 'Even the apples were rotten' 'All the books were lost.'
The most common determiners are the definite article (the) and the indefinite article (a or an).