Center of gravity
From FlightSim
Center of gravity is a specific point in a mass (aircraft in our case) at which, for many purposes, the system's mass behaves as if it were concentrated.
As a simple illustration, when balancing a pencil on your finger, the finger rests at a point which, longitudinally, is below the actual CG. Since the mass of a pencil is usually evenly distributed, resulting in the CG being inside the center of the pencil directly above the finger.
In an aircraft, we are primarily concerned with the fore/aft weight distribution and the resulting fore/aft location of the CG. Changing the number (and positions of) people on board, baggage loaded, fuel on board, even the amount of oil and location of the battery have their effects on CG.
An aircraft is generally certified with a specific range of CG locations, usually near or in the cabin, which are depicted in, and can be calculated by use of, the aircraft manual, along with knowing the weight of each individual person, piece of baggage, etc.
Outside the certified CG range, the aircraft may not handle well, and may not have an adequate range of control (especially in the pitch axis) for safe flight.
See also the center of gravity (aircraft) entry on Wikipedia.
