Orbital inclination

angle between a reference plane and the plane of an orbit

Orbital inclination is a term for the angle between two orbital planes. It is often used to describe the movement of celestial bodies such as stars and planets. In that context, one of the planes is the standard, or reference. Many stars are in a galaxy that has a galactic plane which is used as reference. For a planet's orbit, the inclination is the angle to the ecliptic plane. For a moon, an inclination of 0 degrees means the orbiting body orbits the planet in its equatorial plane, in the same direction as the planet rotates.

A diagram of Keplerian orbital elements.