Calibrated airspeed (CAS) is the speed shown by a conventional airspeed indicator after correction for instrument error and position error. Most civilian EFIS displays also show CAS. At high speeds and altitudes, calibrated airspeed is further corrected for compressibility errors and becomes equivalent airspeed (EAS).
When flying at sea level under International Standard Atmosphere conditions (15°C, 1013 hPa, 0% humidity) calibrated airspeed is the same as equivalent airspeed and true airspeed (TAS). If there is no wind it is also the same as ground speed (GS). Under any other conditions, CAS may differ from the aircraft's TAS and GS.
Calibrated airspeed in knots is usually abbreviated as KCAS, while indicated airspeed is abbreviated as KIAS.
Read more about Calibrated Airspeed: Practical Applications of CAS, Calculation From Impact Pressure