leftover vs residual

leftover and residual both are nouns.

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

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

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

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

leftover (noun)	residual (noun)
a small part or portion that remains after the main part no longer exists	(often plural) a payment that is made to a performer or writer or director of a television show or commercial that is paid for every repeat showing
	something left after other parts have been taken away

leftover (adjective)	residual (adjective)
not used up	relating to or indicating a remainder