dumb newbie question....

[eazy]

Calibrated airspeed (CAS) is the speed theoretically shown by a simple airspeed indicator that is free from instrument error and position error.

A simple airspeed indicator has only one capsule measuring impact pressure (pitot - static differential). CAS must therefore be defined as a function
of impact pressure alone. The instrument does not "know" the absolute static pressure or the static air temperature. Static pressure
and temperature are therefore defined by convention as standard sea level values. [....]

The definition is based on a model of the air as a compressible fluid. CAS therefore represents true airspeed (TAS) at all subsonic speeds
under the reference conditions, i.e. standard sea level pressure and temperature.

At higher altitudes CAS must be corrected for compressibility error to give equivalent airspeed (EAS), and EAS must in turn be corrected for
density to give TAS. In practice compressibility error is negligible below about 10000 feet and 200 knots.

With the advent of the glass cockpit, even in small aircraft, the mechanical airspeed indicator may become obsolete, being replaced by an air
data computer. An air data computer which has inputs of static pressure and total air temperature as well as impact pressure can compute
EAS and TAS directly, as well as mach number, pressure altitude etc. CAS may also therefore become obsolete.
(Wikipedia)

I know, internet directories can be boring sometimes (when it comes to quizzes)