optimum vs optimal

optimum is a noun but optimal is not a noun.

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

As adjectives, optimum and optimal are synonyms defined as:

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

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

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