Plain English Meaning

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.

Full 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.