Use definitely and definitively when there's no doubt, but save definitely for emphasis and definitively for the final say. If you definitely want to go to a party, but your mom definitively says no, then you aren't going. Quit asking.
Definitely means something is definite — it's unmistakable and clearly defined. It's definitely getting warmer on this planet. It can also mean, "Sure" or "Yes" in response to a question. Would you like cake? Definitely! If you forget how to spell it, just think definite plus -ly (keep the e). Here are some examples of definitely in action:
'No, you don't fail,' he assures her. 'You definitely pass.' (Washington Post)
It's best to wear comfy footwear, and definitely not heels, as their spikes can get stuck in the wooden walkway. (Washington Post)
I definitely felt like a little child again, doing the things I love to do, which is play hockey. (Los Angeles Times)
Definitively is a stronger word — anything definitive is official or conclusive. If a doctor gives you a definitive diagnosis, you know exactly what's wrong. It's not a promise that something will happen; it's a done deal. Just add -ly for definitively (once again, keep the e), to say things like, The court has ruled definitively in favor of the defendant. Here are more examples:
They are obsessed with them—especially the kind that are never definitively solved. (Time)
Since 2007, the government has eluded its responsibility and has now invented this farce of a discovery to definitively seize the property. (Los Angeles Times)
It is hard to say definitively, because Apple didn't provide exact figures for comparison. (Wall Street Journal)
As you can see, while there are definitely similarities between these two words, they are at heart definitively different! And that's final.