Hello, would you please tell me what's the difference between 'definitely' and 'definitively' ? thanks.
I presume you've loooked it up, so you'll forgive me if I don't.

I think something which is definitive is unique. It's characteristics are clearly defined.
It's not necessarily one of a kind, but if a designer's style is definitive, it's easily recognizable and easily distinguished from others.
As an adverb, "The work was definitively done." The details of the workmanship were consistent.

"Definitely" is something like "positively."
"He is definitely the man I saw yesterday."

There may be some similarities with "definitive."
"In the fog, his features were indefinite - indistinct."
Definitively is an adjective and is described to specify definitely (not replacing the word definitely), e.g. to clarify in a definitive statement, serving to define something as most reliable or complete as in a text, author, criticism or study, e.g. the definitive guide to management .

Definitely is an adverb as in a definite manner, unambiguously. It is also used to express a complete agreement or strong affirmation, e.g. are you starting your diet tomorrow? Definitely!