Indicated Airspeed (IAS): The direct instrument reading obtained from the Airspeed Indicator (ASI), uncorrected for variations in atmospheric density, installation error, or instrument error.

The Airspeed Indicator (ASI) measures the difference between the Static pressure from the aircraft’s Static ports, and the Ram pressure (Dynamic + Static) from the Pitot tube. This difference is the Dynamic pressure, which translates into a reading.

Calibrated Airspeed (CAS) is Indicated Airspeed corrected for instrument and position error.

True Airspeed is Calibrated Airspeed (CAS) corrected for altitude and nonstandard temperature. True Airspeed (TAS), is the speed of an aircraft relative to the air mass in which it is flying. TAS takes into account factors such as altitude and air density, which can affect the aircraft's performance.

Because air density decreases with an increase in altitude, an aircraft has to be flown faster at higher altitudes to cause the same pressure difference between Pitot impact pressure and Static pressure.

Therefore, for a given CAS, TAS increases as altitude increases, or for a given TAS, CAS decreases as altitude increases.

An aircraft cruises on a heading of 180° with a True Airspeed of 100 kt. The wind comes from 180° with 30 kt. Neglecting instrument and position errors, the approximate reading of the Airspeed Indicator will be 100 kt.