A pitch used by a pitcher who wants to drastically change the speed of his offering, usually achieved by gripping the baseball with several fingers (rather than the usual two). The increased friction from more fingers being placed on the ball results in the pitch traveling at a greatly reduced speed. The changeup also often drops during its arc to the plate due to the physics of the friction and the ball's spin. The changeup often looks like a fastball when it leaves the pitcher's hand, thus increasing the likelihood of upsetting the batter's timing with this change-of-pace pitch.
A changeup can be a devastating pitch, especially for fastball pitchers who have above-average speed. An effective changeup totally destroys a batter's timing because after a fastball or two or three, the ball often arrives at the plate with the same spin as a fastball, but traveling 10-20 mph slower. Many pitchers' changeups often dip as they arrive at the plate, which is when you hear a commentator saying that the ball "disappears" as it gets to the plate.
Most pitchers place three fingers on the ball to produce a changeup, many using what is called a circle change, in which a pitcher makes an "O" with his thumb and index finger and wraps the other three fingers around the ball. Some pitchers even place all of their fingers on the ball, almost holding it in their palm until delivery of the pitch. The changeup can be hard to control, which is why relatively few pitchers have mastered it. Successful pitchers who have mastered the changeup include Trevor Hoffman, among the all-time saves leader in baseball history, and Pedro Martinez, who boasted some of the lowest ERAs of all time at the height of his career with the Boston Red Sox.