In logic and philosophy, the word or concept that is being defined (what you want to explain).
From Latin, future passive participle of definire (to define). Literally 'that which is to be defined.'
When you define 'chair' as 'a seat with four legs and a back,' the word 'chair' is the definiendum—it's the target of your definition. This Latin term lets philosophers discuss the structure of definitions with precision.
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