In logic, the words or phrases used to explain or define another word (what the definition is).
From Latin, present active participle of definire. Literally 'that which defines.' Used as a technical term in formal logic.
The definiens is what does the defining—so in 'a chair is a seat with four legs and a back,' the phrase 'a seat with four legs and a back' is the definiens. Medieval logicians loved these Latin terms for talking about how meaning works.
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