decisiveness vs decisive

decisiveness is a noun but decisive is not a noun.

Word	Noun	Adjective	Verb	Adverb
decisiveness	Yes	No	No	No
decisive	No	Yes	No	No

decisiveness (noun)	decisive (noun)
the trait of resoluteness as evidenced by firmness of character or purpose
the quality of being final or definitely settled

decisiveness (adjective)	decisive (adjective)
	determining or having the power to determine an outcome
	characterized by decision and firmness
	forming or having the nature of a turning point or crisis
	unmistakable