Definite and definitive obviously come from the same root, but they do have separate, differentiated meanings and I don’t think they should be allowed to blur into one another or swap meanings. We have these different words for a reason, and that reason is clarity. There’s getting to be a theme here, as I said a similar thing about refute and rebut, didn’t I!
Definite means clearly stated, decided – not doubtful or vague. It means clearly true or real, and as well as these more intangible meanings, it also means having exact and discernible physical limits. The definite article is “The”. “There is definite evidence that this pipe was not sealed properly”.
When something is definitive, it is done or reached with authority and decisively – and in extension, it refers to the most authoritative of its kind: usually a document or book. “We have found the definitive answer to the meaning of life”, “This is the definitive edition of Ms Broomfield’s great work”.
Oddly, a definitive stamp is one for general use, not commemorative or special. I did not know that.
You can find more troublesome pairs here and the index to them all so far is here.