A definitive answer is a final one. A definitive decision by a court of law is one that will not be changed. A definitive translation is one everyone turns to as correct. Definitive means authoritative, conclusive, final.
Be careful not to confuse definitive with definite. Definite means clearly defined. You can have a definite opinion that JFK could not have been assassinated by a lone gunman, but the definitive interpretation remains that of the Warren Commission Report, which held there was only one.