Plain English Meaning
How fast the airplane is really moving through the air around it, after accounting for the fact that thinner air at higher altitudes makes the airspeed gauge read low.
Full Definition
The actual speed of the aircraft through the surrounding air mass, corrected for the effects of air density (altitude and temperature). It differs from indicated airspeed because the airspeed indicator is calibrated for standard sea-level conditions, and as altitude or temperature increases, the air becomes less dense and the indicator reads lower than the aircraft's real speed through the air.