optimal vs optimum

optimal is not a noun while optimum is a noun.

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

As adjectives, optimal and optimum are synonyms defined as:

optimal and optimum: most desirable possible under a restriction expressed or implied

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

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