Plain English Meaning
A short three-letter code that names a specific navigation aid, like a localizer, so pilots can recognize it on charts and confirm by Morse code that they have tuned the right station.
Full Definition
A three-letter identifier assigned to a navigation facility, such as the localizer of an Instrument Landing System (ILS), used to uniquely identify that facility on charts, in flight plans, and in its Morse code audio identification. For an ILS localizer, the designator is preceded by the letter 'I' (transmitted as the Morse 'I-' prefix) when broadcast as the station identifier.
Context Anchor
Seen when tuning and identifying the localizer for an instrument landing system approach.