Definition
The localizer is the lateral (left/right) guidance component of an Instrument Landing System (ILS). It transmits a directional radio signal aligned with the runway centerline, allowing a properly equipped aircraft to track that centerline precisely during an instrument approach. The localizer provides only horizontal guidance; vertical guidance, when available, comes from the glideslope.
Plain English
A radio signal that tells the pilot whether the aircraft is left or right of the runway centerline during an instrument approach. The cockpit indicator shows the pilot how to steer back onto the line that points straight down the runway.
Context Anchor
Seen on instrument approach charts and in IFR alternate planning, especially when deciding whether an alternate airport has an approach that meets the required minimums.
Derivation
From 'localize' — to fix or pinpoint a position. The localizer 'localizes' the aircraft onto a single, narrow course aligned with the runway.
Why Pilots Care
Maintaining the localizer ensures the aircraft stays aligned for a safe landing; straying too far can result in a missed approach or unsafe conditions.
Intuition Check
LOC does not mean location here. It means localizer: a runway-alignment signal that guides you left or right, not up or down.
Example Sentence 1
Cleared for the LOC Runway 27 approach, the pilot tracked the localizer inbound and used stepdown fixes for descent.
Example Sentence 2
During the approach briefing the crew reviewed the LOC frequency and course.