Plain English Meaning
A radio transmitter beside the runway that sends out a beam telling the aircraft whether it is descending at the right angle to land. The cockpit instrument shows the pilot if they are too high, too low, or right on the slope.
Full Definition
A ground-based radio transmitter, part of an Instrument Landing System (ILS), that radiates a directional signal in the ultra-high frequency band (329.15--335.0 MHz) to define the correct vertical descent path to a runway. The aircraft's glideslope receiver interprets this signal and displays whether the aircraft is on, above, or below the prescribed , typically about .