Definition
A chart provided in an aircraft's Pilot's Operating Handbook that shows the approved combinations of total weight and center of gravity location. The chart displays an enclosed area, called the envelope, bounded by the forward and aft CG limits and the maximum and minimum allowable weights. If the calculated weight and CG point falls inside the envelope, the aircraft is loaded within limits and is legal and safe to fly.
Plain English
A graph in the aircraft manual with weight on one axis and balance point on the other. You plot your loaded aircraft's weight and balance point on it. If the dot lands inside the marked area, you're good to go. If it lands outside, you need to rearrange the load.
Context Anchor
Seen during preflight weight-and-balance calculations, especially when using the graph method in the Pilot’s Handbook of Aeronautical Knowledge.
Derivation
Envelope here comes from the mathematical and engineering sense of a boundary that encloses all acceptable values. It is not the paper kind of envelope. The idea is that anything inside the line is safe, anything outside is not.
Why Pilots Care
Keeps the aircraft controllable and stable by confirming the center of gravity stays inside certified limits before flight.
Analogy
Think of the envelope as a fenced-in area on the graph. If your loading point lands inside the fence, it is allowed; if it lands outside, the loading must be changed.
Intuition Check
Envelope does not mean a paper envelope here. It means the boundary of the allowed loading area on the graph.
Example Sentence 1
After loading three passengers and full fuel, she plotted the numbers on the CG envelope graph and confirmed the aircraft was within limits.
Example Sentence 2
Using the CG envelope graph made it easy to see how adding baggage would move the balance point closer to the aft limit.