I never knew how much people mixed up this word - DEFINITELY - with a whole different word - DEFIANTLY - until I started reading fanfiction/original fiction a long time ago. I just want to point out that these are TWO completely different words with TWO completely different meanings.
At first it didn't bother me but, I'm seeing this more and more often now and I've read some works that have been BETA'd and the word still wasn't corrected. So, let me help clarify the meanings of these two word and give an example of how to use the words correctly in sentences.
DEFINITELY adverb : without doubt
DEFIANTLY adjective : rebelliously: in a rebellious manner
CORRECT WAY TO USE THE WORD DEFINITELY :
I am definitely going to kick his butt after the party is over.
I am defiantly going to kick his butt after the party is over.
CORRECT WAY TO USE THE WORD DEFIANTLY :
The girl held the wolf's gaze defiantly, refusing to be intimidated by the ferocity of it's growls.