residuum vs residual

residuum and residual both are nouns.

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

As nouns, residuum and residual are synonyms defined as:

residuum and residual: something left after other parts have been taken away

Other synonyms of residuum include balance, remainder, residue, rest.

residuum (noun)	residual (noun)
something left after other parts have been taken away	(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

residuum (adjective)	residual (adjective)
	relating to or indicating a remainder