Precession
Definition
The slow circling of a spinning axis — a tilted spinning top does not fall over; instead its axis sweeps around in a cone.
Technical definition
A torque perpendicular to the angular momentum vector changes L's direction, not its magnitude (dL/dt = τ), so the axis sweeps a cone at rate Ω = τ/L.
Example
Earth's axis precesses once every ~26,000 years, slowly changing which star is the 'North Star' — today Polaris, in about 12,000 years Vega.
Common confusion
Precession is not the spin itself, nor a wobble from imbalance. It requires a torque (like gravity on a tilted top); a torque-free symmetric top keeps its axis fixed.