Official Aviation Definition
Calibrated Airspeed (CAS) is the indicated airspeed (IAS) corrected for pitot-static installation and instrument position errors. At low speeds and high angles of attack, position errors can be significant, making CAS a crucial reference for performance data.
In Simple Words
The speed shown on the gauge corrected for minor sensor inaccuracies caused by airflow around the nose or fuselage.
Real-World Flight Example
The aircraft flight manual specifies landing reference speed (Vref) in calibrated airspeed to account for flap position error.