"Definitive" means that whatever your talking about is the last word on its subject matter. Its usual context is "the definitive work", which means a book or publication that is forever after the reference and source material for any other publications or research on that subject. To write the definitive work on a subject, you either need to be an incredible expert or to be the first to study it, but ideally both.

Everything2 is the definitive reference for everything Or it will be, someday.