In cricket, a topspinner is a delivery where the ball comes out the top of the bowler's hand, causing it to spin in the direction of travel. The ball tends to dip in flight, but when the ball bounces, it bounces higher than normal and travels quicker than it would have done with no spin. Consequently, the batsman is more likely to hit the ball in the air, increasing the chances of him being caught out. However, due to the forward spin, a topspinner will occasionally "shoot through", with very low bounce.
A topspinner is generally bowled by a leg spinner or an off spinner. Because the spin is entirely in the direction the ball is travelling in, the ball does not change direction when it bounces as a normal spin delivery would. This can trick the batsman, who may nick the ball behind the stumps, and be caught out by the wicket-keeper or one of the slip fielders.
See also