either complete randomness and disorder or else the primal nothingness that predates existence, depending on the context.

Beyond that, I don't think its possible to define it in terms of a coherant power due to its inherent, well, randomness, becuase humans can't comprehend true randomness.

So just "powers that don't follow the same rules as everyone else" is the closest I can think of in Buu's case.

(Look at Discord, too: He's a Spirit of chaos but if you look at it his powers are't truly random he's just a moderately powerful reality warper.)