A kickback is any of the two 2-glider collisions resulting in a single glider travelling in the opposite direction to one of the original gliders. In a 90-degree kickback, the two gliders collide at right angle, while in a 180-degree kickback they are head-on. Both output gliders are one half-diagonal away from the lane of one of the inputs.
The 90-degree kickback is important in the original proof of the existence of a universal constructor (using an elbow ladder) and in Bill Gosper's total aperiodic, as well as a number of other constructions and glider syntheses. Thus the term kickback reaction may also refer to the 90-degree one specifically. The 180-degree kickback is rarely used in signal circuitry or in self-supporting patterns like the Caterpillar or Centipede, because it is generally less easy to arrange.