In terms of ball games, topspin is defined as spin about a horizontal axis perpendicular to the direction of travel, where the top surface of the ball is moving forward with the spin. Under the Magnus effect, topspin produces a downward swerve of a moving ball, greater than would be produced by gravity alone, and backspin has the opposite effect. Likewise side-spin causes swerve to either side as seen during some baseball pitches, e.g. leg break.
The overall behaviour is similar to that around an airfoil (see lift force) with a circulation which is generated by the mechanical rotation, rather than by airfoil action.
It is named for Gustav Magnus, the German physicist who investigated it. The force on a rotating cylinder is known as Kutta-Joukowski lift, after Martin Wilhelm Kutta and Nikolai Zhukovsky (or Joukowski) who first analyzed the effect.
This is a great share. I've known that this existed in some fashion but never knew what to call it. It makes me think of baseball and how a normal fastball pitch is so different from a knuckle ball. And to think it has non-sport applications is just amazing.