A radio navigation aid which provides vertical guidance during an approach; generally used as part of an ILS installation.
A glideslope (sometimes abbreviated GS) is a ground-based radio-navigation aid used for vertical guidance during an approach; it forms the vertical component of an ils, with lateral guidance provided by the localizer. The vertical guidance provided by the glideslope is what makes a full-up ILS approach a precision-approach (whereas a localizer-only approach, with only lateral guidance, is a non-precision-approach).
The descent profile defined by the glideslope is known as the glidepath; most glideslope installations guide aircraft down a 3° glidepath, but a few are steeper (generally necessitated by nearby high obstructions, natural and/or manmade), with at least two published approaches (both of them in the mountainous part of switzerland) having glidepaths with slopes of 6° or greater.
A glideslope transmitter consists of an antenna array which transmits in two narrow beams - one slightly above the desired glidepath and one slightly below it - which use the same radio frequency, but pulse at different rates. An aircraft's glideslope receiver listens to the combined signal from the two beams; if the overall signal pulses at the rate of the above-glidepath beam, the aircraft is too high (and the pilot's flight-instruments tell them to fly further down), and vice versa.
Unlike the localizer array (which is typically located beyond the far end - from the perspective of the approaching aircraft - of the runway being approached), the glideslope antennae are almost always situated alongside the runway itself, about 1,000 feet downrange from the near end; this is because, in conditions of extreme poor visibility, an aircraft may have to follow the glideslope signal right down to the runway surface, and it is generally considered preferable to land a short distance down the runway rather than overflying the entire runway and touching down at, or beyond, the far end. Essentially the only significant exceptions to this rule of glideslope antenna placement are a very few airports for which terrain considerations preclude a safe straight-in approach, and which are nevertheless heavily-enough-used to warrant a full ILS rather than merely a localizer; in these rare cases, the glideslope and localizer are mounted off-airport (typically at the same location, as land has a tendency to be expensive to acquire), and a pilot shooting an approach must, at some point, go off the ILS and fly visually to the runway. Such a system is known as an instrument guidance system (IGS), an unusual subtype of ILS.