a baseball pitch thrown with little speed or spin
a baseball pitch thrown with the fingernails against the seams of the baseball (so named because the batter can see the pitcher's knuckles as the ball is thrown, not because the knuckles are against the ball) so as to minimize backspin
a type of pitch thrown by a pitcher where the ball does not rotate as it moves towards home plate
This pitch does not spin in flight, but it turns as the air picks up on the seams of the ball. It will usually slide as it reaches the batter.
A knuckleball (or knuckler for short) is a baseball pitch thrown so as to minimize the spin of the ball in flight. The lack of spin causes alternating vortices over the stitched seams of the baseball and produces an erratic, unpredictable motion. This makes the pitch difficult for batters to hit, but also difficult for pitchers to control.