Two i's. Related to "finite." I realize that "definitely" suffers from the murky vowel sounds so common in English, but if you remember "finite," it shouldn't be hard to spell.
For some reason, people feel compelled to spell it "definately." That would be bad enough, but then they find themselves in another path and the letters swap and it comes out as "defiantly." And that, my friends, means something very different.
"Sounds great! I will definitely be there." (With bells on!)
"Sounds great! I will defiantly be there." (With my bazooka!)