Plain English Meaning
A mode where the autopilot or flight director takes over the up-and-down part of an ILS approach, keeping the aircraft on the correct descent angle to the runway.
Full Definition
A flight director and autopilot mode that captures and tracks the glideslope signal of an ILS (Instrument Landing System), commanding the pitch needed to follow the published descent path to the runway. Once armed, the system automatically captures the glideslope as the aircraft intercepts it from below and then flies the descent guidance until disengaged or until the approach is completed.