Wikipedia
Indicated airspeed (IAS) is the airspeed read directly from the airspeed indicator on an aircraft, driven by the pitot-static system. It uses the difference between total pressure and static pressure, provided by that system, to either mechanically or electronically measure dynamic pressure. The dynamic pressure includes terms for both density and airspeed. Since the airspeed indicator cannot know density, it is by design calibrated to assume the sea level standard atmospheric density when calculating airspeed. Not least since the actual density will vary considerably from this assumed value as the aircraft changes altitude, IAS varies considerably from true airspeed (TAS), the relative velocity between the aircraft and the surrounding air mass. Calibrated airspeed (CAS) is the IAS corrected for instrument and position error.
An aircraft's indicated airspeed in knots is typically abbreviated KIAS for " Knots-Indicated Air Speed" (vs. KCAS for calibrated airspeed and KTAS for true airspeed).
The IAS is an important value for the pilot because it is the indicated speeds which are specified in the aircraft flight manual for such important performance values as the stall speed. These speeds, in true airspeed terms, vary considerably depending upon density altitude. However, at typical civilian operating speeds, the aircraft's aerodynamic structure responds to dynamic pressure alone, and the aircraft will perform the same when at the same dynamic pressure. Since it is this same dynamic pressure that drives the airspeed indicator, an aircraft will always, for example, stall at the published indicated airspeed (for the current configuration) regardless of density altitude or true airspeed.
Furthermore the IAS is specified in some regulations, and by air traffic control when directing pilots, since the airspeed indicator displays that speed (by definition) and it is the pilot's primary airspeed reference when operating below transonic or supersonic speeds.
Usage examples of "indicated airspeed".
He finally realized he was passing three hundred knots indicated airspeed in his F-16, still in zone-five afterburner--and he still had his landing gear down.
That should give you about ninety miles per hour of indicated airspeed.
Even at 500 feet and an indicated airspeed of 325 knots, it hadn't been hard for Captain Archer C.
He was flying on a course of 89 degrees magnetic at an indicated airspeed of 140 knots.
The Globemaster was slower now, barely two hundred thirty knots of indicated airspeed, and even with flaps and slats extended, and even with a computer-aided flight-control system, it made for a ride that wallowed on one hand and jerked on the other, something that changed from one second to the next.