An air speed indicator (ASI) is a device for measuring the forward speed of the aircraft. The ASI uses the aircraft pitot-static system to compare pitot and static pressure and thus determine forward speed.

Airspeed is usually measured (and indicated) in knots (nautical miles per hour) although other units of measurement are sometimes encountered.

On older aircraft, airspeed is usually indicated to the pilot on a graduated scale over which a pointer moves (left hand picture below.

In modern aircraft, it is usually indicated on a speed tape which forms part of the Electronic Flight Instrument System display (left hand side of right-hand picture below).