What is "airspeed indicator"

n. (context aviation English) An instrument that displays an aircraft's current airspeed.

The airspeed indicator or airspeed gauge is an instrument used in an aircraft to display the craft's airspeed, typically in knots, to the pilot. In its simplest form, an ASI measures the difference in pressure between the air around the craft and the increased pressure caused by propulsion. The needle tracks pressure differential but the dial is marked off as airspeed.