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.

Definitions of definitive

adj clearly defined or formulated

explicit, expressed
precisely and clearly expressed or readily observable; leaving nothing to implication

adj of recognized authority or excellence

“the definitive work on Greece”
authoritative, classic, classical
established or well-known or widely recognized as a model of authority or excellence

adj supplying or being a final or conclusive settlement

“a definitive verdict”
forming an end or termination; especially putting an end to doubt or question

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