Nutation
Definition
The small nodding wobble on top of precession — a spinning top's axis traces tiny loops as it circles around.
Technical definition
Nutation is the oscillation of the tilt angle in rigid-body motion, a small nodding at higher frequency than the precession, described by the Euler angles.
Example
Earth nods by about 9 arcseconds with an 18.6-year period, driven by the Moon's orbit — a tiny nod riding on the 26,000-year precession.
Common confusion
Nutation is often confused with precession. Precession is the slow circling of the axis; nutation is the small back-and-forth nodding of the tilt angle during that circling.