an action, event etc that is decisive has a big effect on the way that something develops
someone who is decisive is good at making decisions quickly and with confidence [≠ indecisive]:
a decisive leader
a talent for quick decisive action
definite and clear in a way that leaves no doubt
The answer was a decisive no.
—decisiveness noun [uncountable]
Yet again, we have failed to act decisively.
the speed and decisiveness of his victory