Plain English Meaning
The top speed the airplane is allowed to fly. Going faster than this can break the airplane.
Full Definition
The maximum airspeed at which an airplane is certificated to operate, marked by a red radial line on the airspeed indicator. Flight at any speed above VNE risks structural failure, control surface flutter, or other damage that may render the aircraft uncontrollable. VNE is established by the manufacturer during certification and is published in the Pilot's Operating Handbook.
Context Anchor
You will see VNE on the airspeed indicator and in the airplane’s operating handbook. In a spiral dive, airspeed can build quickly toward VNE.