Off-speed pitch

In baseball, an off-speed pitch is a pitch thrown at a slower speed than a fastball. Breaking balls and changeups are the two most common types of off-speed pitches. Very slow pitches which require the batter to provide most of the power on contact through bat speed, are known as "junk" and include the knuckleball and the Eephus pitch, a sort of extreme changeup. The specific goals of off-speed pitches may vary, but in general they are used to disrupt the batter's timing, thereby lessening his chances of hitting the ball solidly or at all.

Virtually all professional pitchers have at least one off-speed pitch in their repertoire. Despite the fact that most of these pitches break in some way (for instance, horizontally, vertically, gradually, or late in their trajectory), batters are sometimes able to anticipate them due to hints that the pitcher gives, such as changes in arm angle, arm speed, or placement of fingers. It is unknown whether off-speed pitches affect batting averages.

...
Wikipedia