remnant vs residuum

remnant and residuum both are nouns.

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

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

remnant: 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 remnant include balance, remainder, residual, residue, rest.

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