optimise vs optimum

optimise is not a noun while optimum is a noun.

Word	Noun	Adjective	Verb	Adverb
optimise	No	No	Yes	No
optimum	Yes	Yes	No	No

optimise (noun)	optimum (noun)
	most favorable conditions or greatest degree or amount possible under given circumstances

optimise (adjective)	optimum (adjective)
	most desirable possible under a restriction expressed or implied

optimise (verb)	optimum (verb)
act as an optimist and take a sunny view of the world
make optimal; get the most out of; use best
modify to achieve maximum efficiency in storage capacity or time or cost