nutation

A nutation is a movement of the direction of a body's axis of rotation, such that each of the two points where the axis emerges from the body describes a circle.

For Earth, the term precession is used for a long, regular cycle of Earth's axial tilt each cycle lasting about 26000 years (precession of the equinoxes). By definition, this could be called a nutation, but for Earth, the term nutation is generally used for what could be termed small irregularities in this precession (effectively consisting of the overall change preceding slightly faster or slower), and other smaller changes in the axial tilt (e.g., the Chandler wobble) caused by shifting portions of Earth's mass and by gravitational interactions with other astronomical bodies. The Moon is a major contributor to Earth's nutation.

Earth's nutation is sufficiently significant (spanning several arcseconds) that it must be taken into account in determining accurate and precise coordinates of a celestial object, and is particularly required regarding astrometry as well as the determination of orbits of solar system bodies.