Plain English Meaning

It's the shared navigation mode that lets one needle in the cockpit show the airplane's position relative to either a VOR ground station or the runway centerline signal of an ILS, depending on which one is tuned.

Full Definition

A combined navigation receiver and indicator mode that displays course guidance from either a VOR station (for en route and terminal navigation) or a localizer (for the lateral course of an ILS approach). On an HSI or course deviation indicator, the same needle and course selector are used for signal types, with the receiver automatically interpreting the tuned and displaying accordingly.