Definite words with the defining root. Definitely.

You might notice a similarity between the following words:

Define
Definition
Defining
Definite
Definitely

They’re all based on the same root words and in every word, the roots are spelled the same way.

Defin(i)

So, in the same way you would not write “defination,” you should not write “definately.”

from Grammarly Blog
http://www.grammarly.com/blog/2015/definite-words-with-the-defining-root-definitely/