Lesson
Center of Gravity, Base of Support, Line of Gravity
CoM: The point where all the mass of a body is concentrated OR
The point about which a body would balance without a 
tendency to rotate (geometric center of body)
At the CoM, All the linear forces acting on the body are balanced, (Sum of Forces = 0) AND
All the rotary forces acting on the body are balanced, (Sum of Moments = 0)
CoG (often used interchangeably with CoM, esp. in uniform gravitational fields):
The point on the body at which all forces acting on it are in equilibrium or balance. (Human CoG located at about second sacral segment). OR The balance point of the body OR
The vertical projection of CoM to the ground (more common useage among biomechanists).
(see: http://www.podiatry-arena.com/podiatry-forum/showthread.php?t=1366)
(more details at: http://www.pt.ntu.edu.tw/hmchai/bm03/bmclinic/stance.htm)
An imaginary vertical line passing through the body’s center of gravity. LoG in ‘optimal’ posture comes close to each joint structure.It is always sought to change a person’s current LoG toward a more ideal posture and body alignment