Calibrated airspeed (CAS) is indicated airspeed corrected for instrument errors and position error (due to incorrect pressure at the static port caused by airflow disruption).
CAS has two primary applications in aviation:
for navigation, CAS is traditionally calculated as one of the steps between indicated airspeed (IAS) and true airspeed (TAS);
for aircraft control, CAS is one of the primary reference points, as it describes the dynamic pressure acting on aircraft surfaces regardless of the existing conditions of temperature, pressure altitude or wind.
The first application has rapidly decreased in importance due to the widespread use of GPS and inertial navigation systems. With these systems, pilots are able to read TAS and groundspeed directly from cockpit displays. This negates the requirement to calculate TAS from IAS with calibrated airspeed as an intermediate step.
The second application, however, remains critical. As an example, at a given weight, an aircraft will rotate and climb, stall or fly an approach to a landing at approximately the same calibrated airspeeds, regardless of the elevation, even though the true airspeed and groundspeed may differ significantly. These V speeds are normally published as IAS rather than CAS so they can be read directly from the airspeed indicator.