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 indicated airspeed and is the value used in many performance charts in the Pilot's Operating Handbook.
Plain English
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.
Context Anchor
Seen in performance charts, aircraft manuals, and FAA discussions of performance speeds.
Derivation
Calibrated' comes from the Latin idea of measuring something against a known standard. A calibrated airspeed is the indicator's reading adjusted to match what the airspeed actually is at that point in flight, once the known errors are accounted for.
Why Pilots Care
Provides the accurate airspeed value needed for performance charts and safe speed decisions.
Intuition Check
Do not assume CAS is simply the number you see on the airspeed indicator. CAS is that number after known installation and instrument errors have been corrected.
Example Sentence 1
After noting 75 knots on the airspeed indicator, the pilot checked the airspeed correction chart in the POH and found the CAS was actually 73 knots.
Example Sentence 2
At low speeds the difference between indicated airspeed and CAS becomes noticeable due to installation error.