Definition
The localizer is the lateral (left/right) guidance component of an Instrument Landing System (ILS). It is a ground-based radio transmitter, sited at the far end of the runway, that radiates a narrow signal aligned with the runway centerline. An aircraft's navigation receiver interprets this signal and drives a course deviation indicator, showing the pilot whether the aircraft is left of, right of, or on the extended runway centerline.
Plain English
The localizer is a radio signal beamed along the runway centerline that tells the pilot if they are tracking straight in toward the runway or drifting off to one side.
Context Anchor
Seen on instrument approach charts and cockpit navigation displays during instrument approaches to a runway.
Derivation
From 'locate' — the localizer's job is to locate the aircraft relative to the runway centerline. The '-izer' ending means 'the thing that does the locating.'
Why Pilots Care
Provides the precise left-right guidance needed to land safely when visibility is too low to see the runway.
Intuition Check
Localizer means left/right guidance only. It does not tell the pilot anything about height above the runway — that is the glideslope's job. The two work together but are separate signals.
Example Sentence 1
Cleared for the ILS Runway 27 approach, the pilot intercepted the localizer and turned to follow it inbound.
Example Sentence 2
LOC data appears in the plan view of the approach plate for every ILS procedure.