Plain English Meaning
How heavy the airplane and everyone in it feel during a maneuver, measured in multiples of normal weight. At 2 G's, you and the airplane effectively weigh twice as much as usual.
Full Definition
The load imposed on an aircraft and its occupants expressed as a multiple of the force of gravity. A G-load of 1 equals the aircraft's normal weight in level flight; a G-load of 2 means the structure and occupants are experiencing forces equal to twice their normal weight. G-load increases during maneuvers that change the aircraft's flight path, such as pull-ups, steep turns, and recovery from dives.
Context Anchor
In the Airplane Flying Handbook spiral dive , G-load matters because a may try to of a fast, steep and accidentally place very on the airplane.