2: a message whose ingenuity or verbal skill or incongruity has the power to evoke laughter [syn: wit, humor, witticism, wittiness]
straight from good ol' dictionary.com, i feel rsl12 has pretty much covered it.
As a sidenote, is humour spelt "humour" or "humor" in the states? I always thought that humor was the correct spelling for the states and humour for the uk but i maybe wrong

Im not up on the differences between the two.