The speed of an aircraft relative to the air through which it's flying, rather than its speed relative to the ground.
From 'air' plus 'speed', a modern compound word created during the aviation era. It's a technical term that distinguishes between ground speed and air speed.
Airspeed versus ground speed is the reason planes can fly backward relative to the earth—a plane flying west at 450 mph airspeed in an east-flowing jet stream of 500 mph will actually move backward relative to the ground while moving forward through the air.
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