Instrument Landing System (ILS)

A precision instrument approach system which normally consists of the following electronic components and visual aids:

Localizer. (See LOCALIZER.)
Glideslope. (See GLIDESLOPE.)
Outer Marker. (See OUTER MARKER.)
Middle Marker. (See MIDDLE MARKER.)
Approach Lights. (See AIRPORT LIGHTING.)

Source: Pilot Contoller Glossary (PCG)

Reference:

14 CFR Part 91
AIM