More about definiendum
Definiendum comes straight from Latin dēfīniendum “to be defined.” In Latin grammar, dēfīniendum is a gerundive, a kind of verbal adjective showing, among other things, obligation or necessity. Dēfīniendum derives from the verb dēfīnīre “to fix the limits of, bound, define,” a compound of the preposition and intensive prefix dē, dē– and the simple verb fīnīre “to mark out or form the boundaries of,” a derivative of the noun fīnis “boundary (of a territory).” Definiendum is a technical term used in lexicography and logic. A similar gerundive, demonstrandum “to be demonstrated,” appears in Q.E.D., an abbreviation of quod erat dēmonstrandum, “what was to be demonstrated,” used at the end of a proof in geometry. Definiendum entered English in the second half of the 19th century.