What would be another word for definitely?

Here are some words that could be used instead of "definitely," depending on the context and desired nuance:

Formal:

* Absolutely

* Certainly

* Positively

* Unquestionably

* Indubitably

* Without a doubt

* For sure

* Assuredly

Informal:

* Totally

* For real

* No doubt

* No question

* Without fail

* Hands down

* In spades

Emphasized:

* Absolutely

* Undeniably

* Beyond a shadow of a doubt

* Without reservation

* Categorically

The best choice will depend on the specific situation and the desired level of formality.