One who or that which determines or decides; in grammar, a word (like 'the', 'a', 'this', 'my') that specifies or limits a noun.
From determine + -er (agent noun suffix). In linguistics, it's a relatively modern term (1920s) borrowed from logic and philosophy to classify a grammatical word class.
English grammar teachers adopted 'determiner' from logicians in the 20th century because it perfectly captures how words like 'the' and 'my' determine which specific noun you mean—one word solved a classification puzzle.
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