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The obliquity (or axial tilt or in context, just tilt) of a planet is the angle between its rotational axis and its orbital axis, or equivalently between its orbital plane and its equatorial plane. Some solar system obliquities:
(These numbers are subject to improvement with newer determinations.) (The angles above 90 degrees indicate the planet rotates retrograde compared to its orbit; subtract such angles form 180° for the angle of the axis irrespective of the direction of rotation; alternately, you could merely state the angle between their axes of rotation and note that they are rotating retrograde.) 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.