A knuckleball or knuckler is a baseball pitch thrown so as to minimize the spin of the ball in flight, causing an erratic, unpredictable motion. The air flow over a seam of the ball causes the ball to transition from laminar to turbulent flow. This transition adds a deflecting force on the side of the baseball. This makes the pitch difficult for batters to hit, but also difficult for pitchers to control and catchers to catch; umpires are challenged as well, since following the path of the ball makes it difficult to call balls and strikes.
This pitch is becoming rare due to scouts trying to find the fastest pitchers rather than the most accurate.