a.

1. 决定性的，决定的
   a decisive moment in his career

   他事业上的关键时刻

2. 确定的
   He gave a decisive answer.

   他给了明确的答复。

3. 坚决的，果断的
   A decisive person is needed to deal with the situation.

   应付这一局面需要一个坚决果断的人。

adj.

1. settling an issue quickly and effectively:
   a decisive battle.
2. able to make decisions quickly.