In baseball, a slider is a breaking ball pitch that tails laterally and down through the batter's hitting zone. It is thrown at a speed that is lower than a fastball, but higher than the pitcher's curveball.
The break on the pitch is shorter than that of the curveball, and the release technique is 'between' those of a curveball and a fastball. The slider is similar to the cutter, a fastball pitch, but is more of a breaking ball than the cutter. The slider is also known as a yakker or a snapper.