Definitive

most reliable or complete, as of a text, author, criticism, study, or the like:
the definitive biography of Andrew Jackson.
serving to define, fix, or specify definitely:
to clarify with a definitive statement.
having its fixed and final form; providing a solution or final answer; satisfying all criteria:
the definitive treatment for an infection; a definitive answer to a dilemma.
Biology. fully developed or formed; complete.
a defining or limiting word, as an article, a demonstrative, or the like.
Philately. a stamp that is a regular issue and is usually on sale for an extended period of time.
Compare commemorative (def 2).
Contemporary Examples

J. Crew Model Armando Cabral: Fashion’s Exotic Boy-Next-Door Robin Givhan November 26, 2012
What Does Ron Paul Have Against Gary Johnson? Ben Jacobs May 14, 2012
The Wrong Case Conor Friedersdorf July 26, 2009
Predator Doctors Take Advantage of Patients With ‘Chronic Lyme’ Scam Russell Saunders September 18, 2014
A Noble Failure: Woodrow Wilson’s Presidency Considered Michael Kazin September 7, 2013

Historical Examples

By Violence John Trevena
The Diplomatic Correspondence of the American Revolution, Vol. IX Various
Three Contributions to the Theory of Sex Sigmund Freud
The Civilization of Illiteracy Mihai Nadin
The Art of War Baron Henri de Jomini

adjective
serving to decide or settle finally; conclusive
most reliable, complete, or authoritative: the definitive reading of a text
serving to define or outline
(zoology) fully developed; complete: the definitive form of a parasite

(of postage stamps) permanently on sale
(as noun) a definitive postage stamp

noun
(grammar) a word indicating specificity of reference, such as the definite article or a demonstrative adjective or pronoun
adj.