A powerball is a special pinball that is made out of a ceramic material as opposed to steel. Several classic pinball machines (such as Kiss), use a powerball for a bonus round.
You might wonder why it would matter what a pinball is made out of? That is simple. Pinball machines are usually loaded with magnets that effect the movement of the normal steel ball (which is why it always seems like the ball heads straight for the gap between your flippers). A ceramic ball is unaffected by the magnets, and allows for a much higher scoring game. Some people even replace all the balls in their machines with powerballs (it makes pinball a lot less frustrating that way).
Replacement powerballs are very expensive, and can easily cost upwards of $100 (USD) per ball (yes thats right, $100 for a little ball).