Plain English Meaning

An invisible sloped beam that tells the pilot whether the aircraft is too high, too low, or right on the correct downward path to the runway.

Full Definition

A ground-based navigation aid that provides vertical guidance to aircraft on final approach, defining the correct descent path to the runway touchdown zone, typically at an angle of about 3 degrees. It is the vertical component of an Instrument Landing System (ILS), working together with the localizer (which provides lateral guidance) to bring the aircraft to the runway in low-visibility conditions.