Official Aviation Definition
Airspeed is the speed of an aircraft relative to the surrounding air. Common classifications include Indicated Airspeed (IAS), Calibrated Airspeed (CAS), Equivalent Airspeed (EAS), and True Airspeed (TAS). Airspeed dictates the aerodynamic forces acting upon wings and control surfaces.
In Simple Words
Unlike a car speedometer that measures how fast you travel across the ground, airspeed measures how fast wind is flowing over the airplane wings.
Real-World Flight Example
During a cruise at FL360 with a strong tailwind, the jet had an indicated airspeed of 260 knots, a true airspeed of 450 knots, and a groundspeed of 540 knots.