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Precession is a cyclical change in an astronomical body's rotation or orbit. The Earth's rotation precesses the direction that the poles point, aligning most closely with different stars as the precession progresses (the precession of the equinoxes, with a period of about 26000 years). In the case of planets or moons this type of precession is called axial precession.
Another type of precession is that of orbits, basically a cycle that shifts the orbit's apoapsis toward different directions as time passes, which is termed an apsidal precession. Well-known distinct examples are the orbit of Mercury, and that of the Moon.
A precession can be quantified by its period or by its rate of rotation, which is essentially the reciprocal of its period. For astronomical bodies, the term precession constant generally refers to the rate of rotation per year. Determining a planet's (axial) precession constant allows its moment of inertia to be calculated.
Other spinning objects besides astronomical bodies can precess, such as a spinning top's axis of rotation rotating so that its point at the top of the top moves in a circular path.