Plain English Meaning
The main navigation source that guides you down the final stretch of an instrument approach. It's the signal you have to be tuned to, identified, and following from the final approach fix to the runway -- not a backup or cross-check.
Full Definition
The navigation aid that provides the principal course guidance for the final approach segment of an instrument approach procedure. It is the facility or signal source the pilot must be receiving and tracking from the final approach fix to the missed approach point, and its identification, tuning, and monitoring requirements are mandatory for the approach to be flown legally.