Learner's definition of DECISIVE
[more decisive; most decisive]
1 : able to make choices quickly and confidently
You must be decisive and persistent to succeed in this competitive field.
I stood there wondering what to do, but my sister was more decisive and immediately went to the phone.
a decisive leader
In emergency situations, one must be able to take decisive action. [=to act quickly and with confidence]
2 : causing something to end in a particular way : determining what the result of something will be
She cast the decisive [=deciding] vote.
The fight ended with a decisive blow.
the decisive battle of the war
The poverty of his childhood played a decisive role in his adult life.
The meeting is seen as a decisive step toward a peace treaty.
◊ A decisive factor is a reason to make a particular choice or decision.
3 : very clear and obvious