Definitively

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

Asiana Flight 214: The Pilot Screwed Up, Right? Clive Irving July 11, 2013
Will Flight Recorder Solve Air France Flight 447 Mystery? Clive Irving May 1, 2011
Russia’s Military Is Already in East Ukraine. Will There Be a Full-Scale Invasion? James Miller August 1, 2014
Harold Ford’s Wall Street Problem Liz Goodwin February 9, 2010
An Absurd Outcome Suzanne Maloney June 12, 2009

Historical Examples

The Minister’s Wooing Harriet Beecher Stowe
Memoirs To Illustrate The History Of My Time Franois Pierre Guillaume Guizot
Encyclopaedia Britannica, 11th Edition, Volume 10, Slice 7 Various
The Companions of Jehu Alexandre Dumas, pre
The Coast of Bohemia William Dean Howells

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.