Pronunciation: (prē-sesh'un),[key] —n. 1. the act or fact of preceding; precedence.
2. Mech.the motion of the rotation axis of a rigid body, as a spinning top, when a disturbing torque is applied while the body is rotating such that the rotation axis describes a cone, with the vertical through the vertex of the body as axis of the cone, and the motion of the rotating body is perpendicular to the direction of the torque.
3. Astron. a. the slow, conical motion of the earth's axis of rotation, caused by the gravitational attraction of the sun and moon, and, to a smaller extent, of the planets, on the equatorial bulge of the earth.
b. See precession of the equinoxes.