a way of emphasizing that something is true and that there is no doubt about it. I definitely remember sending the letter.

There are two meanings listed in OED's entry for the adverb definitely. See 'Meaning & use' for definitions, usage, and quotation evidence.

definite · ​. sure or certain; unlikely to change · ​. easily or clearly seen or understood; obvious synonym clear · ​[not before noun] (of a person) sure that ...

Certainly usually means 'without doubt' or 'definitely', and is used to show that you strongly believe something or to emphasize that something is really true.

without any doubt; certainly: I don't like that place – I'm definitely not going back there. "Are you really going to quit your job?" "Definitely!"

definitive · ​final; not able to be changed. a definitive agreement/answer/statement; The definitive version of the text is ready to be published. · ​[usually ...

​something that you are certain about or that you know will happen; somebody who is sure to do something. 'We're moving our office to Glasgow.

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Certainly is more formal and usually means “without doubt” or “definitely,” and is used to show that you strongly believe something or to emphasize that ...