Plain English Meaning
It is the airspeed shown on the cockpit gauge after fixing the small built-in errors caused by the instrument itself and by where the pressure sensor sits on the airframe. Think of it as a cleaned-up version of indicated airspeed.
Full Definition
Calibrated airspeed (CAS) is the indicated airspeed of an aircraft corrected for instrument error and for position (installation) error of the static pressure source. It is the airspeed the pitot-static system would read if those built-in errors were removed. Calibrated airspeed equals true airspeed in standard atmosphere at .