Definitely and defiantly are both adverbs, but trust us: the similarities end there. You use definitely when you want to describe something clearly, unequivocally, or without a doubt.
You use defiantly to describe something or someone that's challenging or brazenly resistant to something. Most mistakes with these two words arise when writers mean to say definitely but accidentally say defiantly.
So remember: if you put an "A" in definitely, then you're definitely not getting an "A" in English.