Other forms: definienda
Definiendum is a fancy term for a word that is being defined. Definiendum is the definiendum of this blurb.
When you look up the word "diligent" in the dictionary, it's "diligent" that's the definiendum — you'll find it listed there, along with its definition. A word or phrase defined in the context of a sentence is also considered to be a definiendum. If your teacher says, "A parallelogram is a four-sided polygon in which the opposite sides are parallel and equal," the word "parallelogram" is a definiendum.