1. Determining or having the power to determine an outcome
   "cast the decisive vote"; "two factors had a decisive influence" 
2. Unmistakable
   "had a decisive lead in the polls" 
3. Characterized by decision and firmness
   "an able and decisive young woman"; "we needed decisive leadership"; "she gave him a decisive answer" 
4. Forming or having the nature of a turning point or crisis
   - critical

See also: conclusive, crucial, decide, deciding, decisively, decisiveness, definite, determinant, determinative, determining, fatal, fateful, important, peremptory, resolute, unhesitating

Antonym: indecisive

• overheat
• overheated
• overheating
• overindulge
• overindulgence
• overindulgent
• overinflated
• overjealous
• overjoy
• overjoyed
• overkill
• overladen
• overland
• overlap
• overlapping
• overlarge
• overlay
• overlayer
• overleaf
• overleap
• overlie
• overlip
• overload
• overloaded
• overlong