Wiktionary
n. a collection of radio navigation aids used by pilots for approach guidance to a specific airport runway, especially when visibility is limited
Wikipedia
An instrument landing system (ILS) enables aircraft to land if the pilots are unable to establish visual contact with the runway. It does this by way of transmitted radio signals.
It is defined by the International Telecommunication Union as a service provided by a station as follows:
Usage examples of "instrument landing system".
As the seconds ticked by he checked his airspeed and angle of approach on the Vertical Display Indicator in the center of his control panel, carefully lining up the Instrument Landing System cross-pointers on the glowing cursor that represented the Jefferson's location with built-in corrections for wind direction and speed.
Tom scanned his instrument landing system indicator and the altimeter.
It was socked in with rain and we had to land under ILS (instrument landing system).
The dogfight had left him drained, his reactions as automatic as the navigational guidance information from his Instrument Landing System.