Plain English Meaning
The stress the air puts on the airframe when the wings are working harder than usual — for example, in a steep turn or when pulling up sharply.
Full Definition
The force placed on an airplane's structure by the air flowing around it, primarily from the lift the wings must produce to support the aircraft and counter any maneuvering forces. Aerodynamic load increases in turns, pull-ups, and abrupt control inputs because the wings must generate more lift than the airplane's weight, and it is commonly expressed as a load factor (in G's).
Context Anchor
Seen in ground reference maneuvers, steep turns, and any discussion of how and turning affect the airplane.