center of gravity

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The point in or near a body at which the gravitational potential energy of the body is equal to that of a single particle of the same mass located at that point and through which the resultant of the gravitational forces on the component particles of the body acts.
• noun The point of greatest importance, interest, or activity.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics a point, near or within a body, through which its weight can be assumed to act when considering forces on the body and its motion under gravity. This coincides with the center of mass in a uniform gravitational field
• noun any pivotal or central idea or group.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the point within something at which gravity can be considered to act; in uniform gravity it is equal to the center of mass