The center of mass of a system of bodies, such as the solar system. When a comet, for example, is well outside the orbit of Neptune (the farthest major planet), it sees the sun and major planets essentially as a single object of summed mass, and the center of this mass (called the barycenter of the solar system) is offset somewhat from the sun; "original" and "future" orbits of long-period comets are computed for this barycenter, while perturbed, osculating orbits of currently-observed objects in the inner solar system are computed for heliocentric orbits.
The barycenter is the center of mass around which a system of objects rotate. For example, the Earth and the moon both revolve around their barycenter, which is 1707 km inside the Earth along the line between them. Pluto and its moon Charon revolve around their barycenter, which is 700 km inside Pluto along the line between them.