Obliquity (or axial tilt or in context, just tilt) is the angle between a planet's rotational axis and its orbital axis, or equivalently between its orbital plane and its equatorial plane.
Solar system examples:
Both the Moon and Mercury have an obliquity such that the polar regions receive little sunlight.
Tilt erosion is the (slow) reduction of axial tilt, which can be the result tidal forces. It has been suggested that some axial tilt may increase the chances of a planet's habitability, and that tilt erosion could reduce it.