Seam, seam bowling/bowler

A cricket ball is not a perfect sphere. The two hemispheres that make up its leather cover are held together by the stitched seam, which - on a new ball - starts the match standing proud of the surface. If the bowler can cause a delivery to pitch on this seam there is a good chance that it will deviate from its apparent direction, causing the striker to play down the wrong line and possibly snick the ball to the wicket-keeper or one of the close fielders.
A seam bowler is one who attempts to bowl this type of delivery.