Definition: In table tennis, sidespin occurs when the ball is rotating at an angle to its direction of movement. When the ball is rotating at 90 degrees to its direction of motion, it has pure sidespin. When the ball is rotating in the same direction it is moving, it has either topspin or backspin only. Most strokes have a combination of sidespin and either topspin or backspin.
Sidespin will make the ball curve to the left or right in the air. If the left side of the ball is moving in the opposite direction to the motion of the ball, the ball will curve to the left, and vice versa. A ball with sidespin will also tend to bounce a little sideways on the playing surface instead of straight forward, and do the same when hitting a racket.