Orbital inclination (i) is the angle between the orbital plane of an object and a reference plane that passes through the body being orbited. The inclination of a planetary orbit in the Solar System is measured from the ecliptic (the plane of Earth's orbit around the Sun). The inclination of a satellite's orbit is measured from the plane that contains its primary's equator. For binary stars, the inclination is specified relative to the plane of the sky. Orbital inclination is an orbital element.

Axial inclination is the angle between a planet or moon's rotation axis and its orbit; in the case of a star, the reference plane is usually that of the celestial sphere.