According to CAP 696 the following is a definition of the Centre of Gravity (CG): that point on the longitudinal axis of the aircraft through which all of the mass acts vertically downward.

In other words, CG is the point at which an aircraft would balance if it were possible to suspend it at that point. It is the mass center of the aircraft, or the theoretical point at which the entire mass of the aircraft is assumed to be concentrated. It may be expressed as distance from the reference datum, or in percent of Mean Aerodynamic Chord (MAC). The location of the CG depends on the distribution of mass in the aircraft.