Plain English Meaning
The fastest speed the airplane is allowed to fly in normal operations, measured by the airspeed indicator. Flying faster than this risks damaging the airplane or losing control.
Full Definition
VMO is the maximum airspeed, expressed in knots indicated airspeed (KIAS), at which an airplane is permitted to be operated during normal flight. Exceeding VMO can lead to structural damage, control difficulties, or flutter. It is established by the manufacturer and published in the airplane's flight manual.
Context Anchor
Pilots see VMO in the airplane’s limitations, performance information, and airspeed discussions, especially during or -speed operation.