inclination (symbol i)

One of the principal orbital elements used to define an orbit, giving the angle between the orbital plane and a reference plane. For the orbits of planets and comets around the Sun, for example, the reference plane is the ecliptic. For satellite orbits, it is the plane of the equator of the parent planet. The term inclination is also used for the angle between a body's rotation axis and a reference plane, normally the body's orbital plane.