A very high, loopy groundstroke with heavy topspin that arcs well above the net. Technically a valid tactic (the ball bounces deep and high, pushing your opponent back), but it's widely considered annoying to play against. At the junior and recreational level, moonballers are the bane of aggressive players everywhere. It works until it doesn't, and it's never fun to lose to.
"Every ball coming back is a moonball six feet above the net. It's working, but nobody in the stands is enjoying it."