, a "rising fastball" is a fastball
thrown so fast and so hard that it appears to rise as it approaches home plate
in a sudden burst of speed.
Of course, such a motion is physically impossible, as it would break fundamental laws of gravity and conservation of momentum, and therefore is actually somewhat of an optical illusion. The rising fastball does not actually rise, but does experience an unexpected change in its trajectory that gives the appearance of a rising action. This occurs because a fastball (in this case, almost always a four-seamer) is thrown with a large amount of backspin. As the ball approaches the plate, it is actually slowing down, not speeding up, due to the action of air resistance, but because it has so much backspin, it "rises" to a higher trajectory than the batter or an observer would otherwise suspect, even though the ball is not actually rising above a previously attained maximum height.
The only type of pitcher who could truly throw a rising fastball would be a submariner, who throws underhand and thus could actually achieve a rising action. However, this type of pitch is usually not what is being referred to when the term "rising fastball" is employed.