Airspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are: indicated airspeed ("IAS"), calibrated airspeed ("CAS"), true airspeed ("TAS"), equivalent airspeed ("EAS") and density airspeed. During cruising flight at altitudes, airspeeds, and temperatures common for airliners, the four speeds trace a shape that looks like the mathematical square root symbol (√). Starting with indicated airspeed, calibrated is normally very close to the indicated airspeed, while equivalent is normally less than both indicated and calibrated, and true is normally higher than the other three. The measurement and indication of airspeed is ordinarily accomplished on board an aircraft by an airspeed indicator ("ASI") connected to a pitot-static system. The pitot-static system comprises one or more pitot probes (or tubes) facing the on-coming air flow to measure pitot pressure (also called stagnation, total or ram pressure) and one or more static ports to measure the static pressure in the air flow. These two pressures are compared by the ASI to give an IAS reading.

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