A volleyball serve, similar to a knuckleball pitch in baseball. The ball moves unpredictably, possibly swerving or dropping suddenly.
This server can be performed either overhand or underhand. The advantage of an overhand serve is that more power can be applied, since there is no upward component to the initial trajectory. One advantage of the underhand serve is that it requires less strength; another is that opponents almost always underestimate underhand serves (since it is usually one of the first things to go when transitioning from novice to intermediate play).
The key to performing a floater serve is to remove almost all spin from the ball. Strike the ball flat with a stiff wrist and do not follow through with the serve. If serving underhand, for example, the striking hand should not follow past the hand holding the ball.
The erratic trajectory of the floater is caused by air turbulence. The seams in the surface of the volleyball create turbulence in the air, and this turbulence produces low-pressure regions around the ball, especially in its wake. The irregular pressure causes the ball to deviate from a smooth path. If the ball were completely without spin with respect to its trajectory, the trajectory would also be smooth, but this is nearly impossible; hence, the inevitable slight spin causes the erratic movement. A fast spin, on the other hand, causes the turbulence to be "averaged out", leading to a smooth flight.