1.Definition: characterized by decision and firmness

"An able and decisive young woman", "We needed decisive leadership", "She gave him a decisive answer"

2.Definition: determining or having the power to determine an outcome

"Cast the decisive vote", "Two factors had a decisive influence"

3.Definition: forming or having the nature of a turning point or crisis

Related Adjective(s):critical

4.Definition: unmistakable

"Had a decisive lead in the polls"