In astrodynamics, an orbiting body is any physical body that orbits a more massive one, called the primary body. The orbiting body is properly referred to as the secondary body (), which is less massive than the primary body ().
Thus, or .
A system of two orbiting bodies is modeled by the Two-Body Problem and a system of three orbiting bodies is modeled by the Three-Body Problem. These problems can be generalized to an N-body problem. While there are a few analytical solutions to the n-body problem, it can be reduced to a 2-body system if the secondary body stays out of other bodies Sphere of Influence and remains in the primary bodies sphere of influence.