1. For planets, the angle between the plane of the ecliptic and the plane of the planet's orbit or the angle between a line perpendicular to the plane of the ecliptic and the planet's axis of rotation.

For satellites, orbital inclination is expressed relative to the parent planet's orbital plane.

2. In terrestrial magnetism, the angle through which a freely suspended magnet would dip below the horizon in the magnetic north–south meridional plane; one of the magnetic elements.

At the aclinic line (dip equator) the inclination is zero; at either magnetic pole (dip pole) the inclination is 90°.

3. The angle between the plane of the satellite orbit and the earth's equatorial plane.

An inclination angle of less than 90° is referred to as a prograde orbit, while an inclination angle greater than 90° is called a retrograde orbit.