Plain English Meaning
A needle in the cockpit that tells you whether you are too high, too low, or right on the correct downward path to the runway during an instrument approach.
Full Definition
A cockpit instrument display, typically a horizontal needle on the course deviation indicator, that shows the aircraft's vertical position relative to the published glideslope of an Instrument Landing System (ILS) approach. The needle deflects up when the aircraft is below the glideslope and down when above, with the centered position indicating the aircraft is on the correct descent path to the runway.