The term for a fastball thrown by a pitcher who places his fingers on the two seams of the baseball, often resulting in movement of the pitch as more pressure is placed on either finger. Some pitchers have natural movement from this grip, which result in a different path for the baseball, differing noticeably from the traditional four-seam fastball, which usually does not move in either direction.
The two-seam fastball has grown in popularity over the years because it combines fastball speed with some movement, making it even tougher for hitters. This pitch is also often called a "cutter", meaning that it cuts in or away from a hitter. The most effective two-seam fastball is often one that cuts in to a hitter, producing a jammed swing and often breaking the bat of the hitter as the pitch arrives in on his hands. The master of the two-seamer is acknowledged to be Mariano Rivera, one of the greatest relief pitchers of all time.