Definition
A radio frequency in the 108.10 to 111.95 MHz band, on odd-tenth decimal channels (for example 108.10, 108.15, 108.30, 108.35), used by the localizer portion of an Instrument Landing System to provide lateral guidance to a runway centerline. These frequencies share the lower end of the VHF navigation band with VOR frequencies, but ILS localizers are assigned only the odd-tenth decimal channels, while VORs use the even-tenth decimals.
Plain English
It is the specific radio channel a pilot tunes to receive the runway centerline signal during an instrument landing approach. The channel always falls between 108.10 and 111.95 MHz, and the decimal pattern (odd tenths) tells you it is an ILS localizer rather than a VOR.
Context Anchor
You see it on an instrument approach chart and set it in the navigation radio before flying an ILS approach.
Derivation
Localizer comes from 'localize' (Latin locus, meaning place) -- the signal helps the pilot locate the runway centerline. Frequency simply means how many radio waves per second, which identifies the channel.
Why Pilots Care
Selecting the published frequency lets the receiver capture the localizer signal for accurate lateral guidance during low-visibility landings.
Intuition Check
Do not read frequency here as “how often something happens.” In this context, it means the radio channel used to receive the localizer signal. Also, the localizer does not provide all ILS guidance by itself; it gives left-right runway alignment guidance.
Example Sentence 1
Approaching the final approach fix, the pilot confirmed the ILS localizer frequency of 110.30 was tuned and identified before intercepting the localizer course.
Example Sentence 2
Each runway end with an ILS has its own localizer frequency to prevent signal overlap from nearby facilities.