residuum vs leftover

residuum and leftover both are nouns.

Word	Noun	Adjective	Verb	Adverb
residuum	Yes	No	No	No
leftover	Yes	Yes	No	No

As nouns, leftover is a hyponym of residuum; that is, leftover is a word with a more specific, narrower meaning than residuum:

residuum: something left after other parts have been taken away
leftover: a small part or portion that remains after the main part no longer exists

Other hyponyms of residuum include remnant.

residuum (noun)	leftover (noun)
something left after other parts have been taken away	a small part or portion that remains after the main part no longer exists

residuum (adjective)	leftover (adjective)
	not used up