Plain English Meaning
The fastest speed the airplane is allowed to fly right now. In some airplanes this limit changes with altitude, so a moving pointer on the airspeed indicator shows the current limit at all times.
Full Definition
The highest indicated airspeed at which an aircraft may be safely operated, which on certain high-performance airplanes varies with altitude. It is shown on the airspeed indicator by a movable pointer (often called a barber pole) that automatically adjusts as altitude changes, reflecting the lower of the structural limit (Vne or Vmo) or the Mach limit (Mmo) at any given altitude.
Context Anchor
Seen in airspeed indicator markings, aircraft operating limitations, and discussions about different .