1. The angle between the orbital plane of a body and the reference plane centred on the object about which the body is revolving. For planets in the Solar System, the inclination of the orbit is usually given relative to the plane of the Earth’s orbit, the ecliptic. For objects orbiting a planet, the inclination is usually given relative to the planet’s equator. For double stars, the inclination is given relative to the plane of the sky. The inclination is one of the elements of an orbit.
2. The angle at which the rotation axis of a planet or satellite is tilted with respect to the perpendicular to some reference plane. The reference plane is usually the plane of the body’s orbit.