1. The apparent backward movement, or reversal of direction, by outer planets in the solar system.

Retrograde motion is simply an optical illusion, created by the fact that the earth is orbiting the sun much faster than the outer planets are.

2. The clockwise, or east to west, motion of a body, and hence the reverse of direct motion.

As the majority of bodies in the solar-system orbit around their governing bodies; that is, the sun or planet, in direct motion; the occurrence of retrograde motion usually indicates some peculiarity.