The axial tilt of any celestial body is the inclination of its axis from a line perpendicular to the plane of its orbit around its primary. It is also the inclination of its equator from that orbital plane.
Planets with highly tilted axes are problematic for the usual model for the creation of the solar system (the nebular hypothesis). A planet that formed by accretion of matter from a larger spinning disk ought to rotate in the same plane as the disk; its axis should not tilt. Furthermore, the axis of the Sun ought not tilt from the orbital planes of most of its planets—but in fact the Sun's axis is tilted from the ecliptic, and by more than seven degrees.