Astronomy Answers
From the Astronomical Dictionary

[Astronomy Answers] [Dictionary] [AnswerBook] [Universe Family Tree] [Science] [Starry Sky] [Planet Positions] [Calculate] [Colophon]

The description of the word you requested from the astronomical dictionary is given below.

the center of mass

The center of mass of a system is the average of the positions of all objects in the system, each position weighted with the mass of the object. The sum of all the forces from outside the system on the objects of the system is the same as the force on a single particle with the total mass of the system at the center of mass. Forces between the objects in the system cannot influence the center of mass.