definitive

Concise Oxford English Dictionary © 2008 Oxford University Press:
definitive/dɪˈfɪnətɪv/

▶adjective

• 1 (of a conclusion or agreement) done or reached decisively and with authority.

  ■ (of a book or other text) the most authoritative of its kind.

• 2 (of a postage stamp) for general use, not special or commemorative.

▶noun a definitive postage stamp.

– derivatives
definitively adverb.

– origin ME: from OFr. definitif, -ive, from L. definitivus, from definit-, definire (see define).