Definition
The component of an Instrument Landing System (ILS) that provides lateral guidance to the runway centerline. The localizer transmits two overlapping radio signals along the approach path; the aircraft's navigation receiver compares the strength of the two signals and displays the aircraft's position left or right of the extended runway centerline.
Plain English
A radio signal beamed out along the runway centerline that tells the pilot whether the aircraft is lined up with the runway, or drifting to the left or right of it.
Context Anchor
You encounter a localizer when flying or briefing an ILS approach, and you see its guidance on the cockpit course display as you line up with the runway.
Derivation
From 'localize' -- to fix the position of something. The localizer 'localizes' the aircraft relative to the runway centerline by telling the pilot exactly where they are side-to-side.
Why Pilots Care
Provides the horizontal guidance required for safe, low-visibility landings when the runway cannot be seen until late in the approach.
Intuition Check
Do not read “localizer” as a complete position-finding system. In this context, it gives left-right runway alignment guidance; it does not by itself give altitude guidance or a full map position.
Example Sentence 1
After being cleared for the ILS approach, the pilot tuned the localizer frequency and watched the needle center as the aircraft intercepted the final approach course.
Example Sentence 2
Once established on the localizer, the aircraft descended along the glide slope toward the runway threshold.