Plain English Meaning
How fast the aircraft is moving through the air around it — not how fast it is moving over the ground. The wings only know about airspeed; that is what keeps the aircraft flying.
Full Definition
The speed of an aircraft relative to the air mass through which it is moving. Airspeed is distinct from groundspeed and is the speed that matters aerodynamically — it determines lift, drag, and how the aircraft handles. Several specific forms are recognized: Indicated Airspeed (IAS), the speed shown directly on the airspeed indicator; Calibrated Airspeed (CAS), IAS corrected for instrument and position errors; (EAS), CAS corrected for at higher speeds and ; and (), EAS corrected for , representing the actual speed of the aircraft through the surrounding air mass.