Plain English Meaning
CAS is the airspeed reading you'd get if you took the number on the airspeed gauge and cleaned it up by removing the small errors caused by where the sensor sits on the aircraft and tiny inaccuracies in the gauge itself.
Full Definition
Calibrated Airspeed (CAS) is the airspeed shown on the airspeed indicator after it has been corrected for two specific errors: position (or installation) error and instrument error. Position error comes from how airflow behaves around the static port at different airspeeds and aircraft attitudes. Instrument error comes from small mechanical inaccuracies in the airspeed indicator itself. CAS is more accurate than the raw and is the used in many in 's Operating .