Definitely is definitely one of those words that is misspelled a lot. I have lost count of the number of ways I’ve seen it written (even defiantly, believe it or not), but the most popular seems to be “definately” – as in, Defin ate Ely, whoever they are.
If you’re one of the offenders of this crime against literature, here’s a funny image to help you next time. (Alternatively, think of finite, as in, something that has an end. That’s how it’s spelled.)