This is a type of pitch throw in baseball which has a speed and velocity less than that of a fastball. The idea behind this type of pitch is to alter and confuse the timing of the batter. Some common examples are the change-up and breaking ball. The change-up has the same or similar wind-up as a fastball, but when it is delivered, it moves a lot slower than a fastball. This will usually result in the batter swinging too early or mistiming their swing altogether. The speed difference occurs because the ball is released at a higher point in the wind-up than a fastball, which reduces the momentum behind the pitch. A breaking ball will have spin on it that affects the speed. There are many different types of breaking balls, there is no one type.
Another term used for off-speed pitches is "junk balls." They are called this because batters do not like them. When a pitcher throws an off-speed pitch, a batter may sometimes be heard remarking, "stop throwing junk," or something to that effect. However, off-speed pitches are an essential part of the game and as long as pitchers are capable of throwing them and getting batters out, they will continue to be a part of the game.