Airspeed measures how fast the air is flowing over the wings, which dictates the aerodynamic lift and structural limits of the aircraft. It is fundamentally different from ground speed, which is the speed over the Earth's surface. Pilots track indicated airspeed for flying, but calculate true airspeed for navigation.
Heard on the job
Keep your airspeed up on final approach so we don't stall in this headwind.
Insider tip
A plane can have an airspeed of 150 knots but a ground speed of zero if it is flying into a 150-knot headwind.