WordWeb Online

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

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

Antonym: indecisive