What is "two-seam fastball"

n. (context baseball English) A pitch thrown with a grip in the "horseshoe" orientation of the ball, with the rotation at a slight angle resulting in less http://en.wikipedia.org/wiki/Magnus%20effect as compared to a four-seam fastball since only two of the ball's seams cross the flight path; the pitch is delivered with a similar velocity as a four-seam fastball, but has a degree of sink.

A two-seam fastball is a pitch in baseball and a variant of the straight fastball. The pitch has the speed of a fastball, but the general movement of a screwball. It is a close relative to the sinker.