In astronomy, axial tilt, known to astronomers as obliquity, is the angle between an object's rotational axis, and its orbital axis, or, equivalently, the angle between its equatorial plane and orbital plane. It differs from orbital inclination.

Orientation of the axes is established by the right hand rule for both the rotation and the orbital motion. When the fingers of the right hand curl around in the direction of the object's rotation, the thumb points in the direction of its north pole (from which, looking back at the object, it appears to rotate counter-clockwise). Similarly, when the fingers of the right hand curl around in the direction of the object's orbital motion, the thumb points in the direction of the north pole of the orbit (from which the object appears to move counter-clockwise in its orbit). The angle between these two poles is the obliquity. At an obliquity of 0°, the axes point in the same direction.

Because the planet Venus has an axial tilt of 177° its rotation can be considered retrograde, opposite that of most of the other planets. The north pole of Venus is "upside down" relative to its orbit. The planet Uranus has a tilt of 97°, hence it rotates "on its side", its north pole being almost in the plane of its orbit.