Official Aviation Definition
The glideslope is the vertical guidance element of an Instrument Landing System (ILS). It transmits an inclined UHF radio beam (typically at a 3-degree angle) from near the runway threshold, enabling pilots or flight directors to descend smoothly to the runway touchdown zone.
In Simple Words
An invisible radio beam in the sky that shows pilots whether they are descending too high, too low, or right on track to the runway.
Real-World Flight Example
The autopilot captured the ILS localizer at 4,000 feet and followed the glideslope down to minimums in low visibility.