decisiveness has 3 distinct synonyms spread across 2 of its 2 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is indecisiveness.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
the trait of resoluteness as evidenced by firmness of character or purpose
“a man of unusual decisiveness”
the quality of being final or definitely settled
“the finality of death”
Synonyms, antonyms and the broader and narrower relations for decisiveness come from Princeton University’s WordNet. Full sources and licences.