Answer: definitive

AnswerCrossword Clue
DEFINITIVEAbsolute
definitive(of a conclusion or agreement) Done or reached decisively and with authority
definitiveserving to define / a definitive postage stamp
definitive(of a postage stamp) For general use and typically of standard design, not special or commemorative
definitive(of a book or other text) The most authoritative of its kind