1) Center of mass 2) Centre of mass 
From physics, this is the center of mass: the point at which a mass (body) can be considered as being concentrated without altering the effect of the attraction that the earth has upon it. This is the average position ({x^i}) of particles, weighted by their masses ({m_i}): Sum_i{m_i x^i}/Sum_i{m_i}. In mathematical programming, this is usually use....
The common center of mass about which two or more bodies revolve.
The center of mass of a system of objects moving under the influence of their mutual gravity.
The common center of mass about which two or more bodies revolve. 
[n] - (astronomy) the common center of mass around which two or more bodies revolve
barycenter, barycentric The center of mass in any system of celestial objects moving under mutual gravity; used primarily with reference to the earth-moon system.
noun (astronomy) the common center of mass around which two or more bodies revolve