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 ...

(of a person) sure that something is true or that something is going to happen and stating it to other people I'm definite about this.

something that you are certain about or that you know will happen; someone who is sure to do something “Is Sarah coming to the party?” “Yes, she's a definite.” ...

6 дней назад · 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!"

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

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.