It is when instead of:
There are major differences between the two words. You don't want to confuse them.
DEFINITELY is the adverb form of the word DEFINITE. Meaning: unquestionable, decided.
DEFIANTLY is the adverb form of the word DEFIANT. Meaning: Showing Defiance, to defy.
Clearly, two differently spelled words. Two very different definitions.
Now... frequently, I see people confusing DEFINITELY and writing DEFIANTLY in its place. Creating situations like this:
INCORRECT: I defiantly want to go on a date with you. (Meaning, I'd like to defy you by accepting your date request. I probably don't like you very much).
CORRECT: I definitely want to go on a date with you tonight. (Meaning, yes, I'll go on a date with you. I'm just that into you.)
See the problems that occur when the two words are confused? Your entire meaning is changed!
The main reason people use DEFIANTLY instead of DEFINITELY is because spell check doesn't catch it. Although I am not sure why people accidentally put an "A" into a word that has no A.
Which leads to the next probable reason people confuse these words: Many people MISTAKENLY think that "definitely" has an "A" in it, because when we say the word fast, it sort of SOUNDS like there's an "A."
Which is why you'll see another common usage error... people spelling DEFINITELY as DEFINATELY. There's no "ate" in definite.
Please, please, please... check your spelling of the word DEFINITELY... otherwise, you come across as... well, a bit defiant! Is that the impression you want?
What's your English grammar pet peeve!?