remaining vs leftover

remaining is not a noun while leftover is a noun.

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

As adjectives, remaining and leftover are synonyms defined as:

Other synonyms of remaining include left, left over, odd, unexpended.

remaining (noun)	leftover (noun)
	a small part or portion that remains after the main part no longer exists

remaining (adjective)	leftover (adjective)
not used up	not used up