Decisive describes someone who makes their mind up and acts. The manager who picks a plan in five minutes and starts moving people; the friend who says we are going here while everyone else is still reading reviews.
The second half of the word is about events rather than people. A decisive goal, a decisive victory, the decisive moment in a trial — these are the ones that settle the outcome. That is the trap for learners: decisive does not mean important. A decisive battle is the battle that ended the war, and a decisive vote is the one that tipped it. The adverb is decisively, as in they won decisively; the noun is decisiveness; and the opposite, indecisive, is aimed at people far more than at events.
1adjectivedecisiveAble to make choices quickly and firmly.
“A referee has to be decisive, even when the whole crowd disagrees.”
2adjectiveconclusiveSettling the outcome of something; deciding the result.
“Her goal in the last minute proved decisive.”
synonyms: conclusive, deciding
Decisive came to English from Middle French décisif, and English reads it as plainly built: decide plus the -ive ending that means having this quality, the same ending in creative and attractive. So decisive is having the quality of deciding — which is neatly what both its meanings do. One decides for itself; the other decides the outcome for everyone.
Middle French
décisif
English, today
decisive
From Middle French décisif, equivalent to decide + ive.
Etymology from Wiktionary, CC BY-SA