decisive (adj.)

late 14c., "to settle a dispute, determine a controversy," from Old French decider, from Latin decidere "to decide, determine," literally "to cut off," from de "off" (see de-) + caedere "to cut" (from PIE root *kae-id- "to strike"). For Latin vowel change, see acquisition. Sense is of resolving difficulties "at a stroke." Meaning "to make up one's mind" is attested from 1830. Related: Decided; deciding.

"resolute, free from hesitation or wavering," 1790, past-participle adjective from decide. A decided victory is one the reality of which is not in doubt; a decisive one goes far toward settling some issue. Meaning "free from ambiguity or uncertainty" also is from 1790. Related: Decidedly.