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. Pizza is definitely popular in New York City. 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 were both definitively cleared of the charges last year. (Reuters)

Perhaps this book will difinitevely answer some of the enduring questions about Lynch's work. (Los Angeles Times)

The sample size was too small to definitively say it's 100 percent effective. (Scientific American)

As you can see, while there are definitely similarities between these two words, they are at heart definitively different! And that's final.