residuum vs remnant

residuum and remnant both are nouns.

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

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

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

Other hyponyms of residuum include leftover.

residuum (noun)	remnant (noun)
something left after other parts have been taken away	a piece of cloth that is left over after the rest has been used or sold
	a small part or portion that remains after the main part no longer exists