Said when something is too awesome for all other words or word phrases. Can be used as an adjective, noun, or a verb. When you say it, you must exclaim it in a high pitched, enthusiastic voice. If you use it wrong, you're dumb.
The opposite is Oh Never! Which is used when you are describing or reacting to something that is terrible or something that you would never do.
Noun: "She's an Oh Definitely!"
Verb: "I would Oh Definitely her!"
Adjective: "That girl is so Oh definitely!"
Or simply: "OH DEFINITELY!!"
(v.) A combination of two words: define (to assign meaning to a word) and defecate (to take a crap). To assign an arbitrary meaning to a word spontaneously; to fabricate a definition on the spot. In other words: to pull a definition out of your ass. Related: defination (n.)
In a melodious harmony, the word "definate" was, in itself, definated to become a defination.