When at different altitudes, the change in pressure and temperature affects the capacity of an aircraft’s standard system to accurately indicate airspeed as the number of air molecules entering the pilot’s tube is fewer than it would be at sea-level. Consequently, a system of measurement to evaluate the correct airspeed at varying altitudes was developed to give the pilot the aircraft’s true airspeed. Therefore, this term simply refers to the correct airspeed of a plane at altitude.