Precession, also called epicyclic fretting precession, (or more accurately hypocyclic fretting precession since "epicyclic" applies to a round part spinning outside a circle and "hypocyclic" applies to a round part spinning inside a circle) is the process of a round part in a round hole rotating with respect to that hole because of clearance between them and a radial force on the part that changes direction. The direction of rotation of the inner part is opposite to the direction of rotation of the radial force. Fretting between the part and the hole is often a result of this motion. "In machinery, fretting is the micro-motion of tightly fitting parts that superficially appear immobile with respect to each other."