What does "airspeed" measure?

Airspeed is a critical parameter in aviation as it measures the speed of an aircraft relative to the surrounding air. This measurement is significant because an aircraft's performance, including lift, drag, and fuel efficiency, are all influenced by the aircraft's movement through the air.

Understanding airspeed is essential for pilots because they must ensure that they maintain specific airspeeds for safe flight operations, such as during takeoff, cruising, and landing. It directly relates to how the aircraft interacts with the air it is flying through, affecting its control and performance characteristics.

The other options, while related to flight, do not represent what airspeed specifically measures. The speed compared to the ground pertains to ground speed, altitude refers to the vertical distance an aircraft is above sea level, and wind is a factor that can affect aircraft performance but does not define airspeed itself. Thus, the focus on relative motion through the air makes option B the correct definition of airspeed.