The slider is a pitch in baseball somewhere between a curveball and a fastball. The slider is much faster than a curveball, but does not break as much, while it is significantly slower than the fastball.
A good slider is one of the toughest pitches to hit, but it is also one of the hardest pitches on the elbow.
The slider was developed in the 1950s and first came into widespread use in the 60s. Bob Feller was one of the first to master this difficult pitch. Ted Williams said he could have played five more years if not for the slider.
Many people agree that Randy Johnson has the best slider in the game today.