leftover vs residuum

leftover and residuum both are nouns.

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

As nouns, residuum is a hypernym of leftover; that is, residuum is a word with a broader meaning than leftover:

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

Other hypernyms of leftover include balance, remainder, residual, residue, rest.

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

leftover (adjective)	residuum (adjective)
not used up