Definition
On an ILS approach profile view, the published altitude at which the aircraft, while level on the final approach course, intercepts the glide slope from below and begins the descent to the runway. It is depicted on the approach chart with a lightning-bolt symbol at the point where the glide slope meets the level segment.
Plain English
The altitude you fly level at until the glide slope catches up with you. Once you reach it, you start descending down the glide slope toward the runway.
Context Anchor
Seen in the profile view of an instrument approach chart, especially on approaches that use a glide slope.
Derivation
Glide slope is the angled radio beam that guides the aircraft down to the runway. Intercept means to meet or join. So the glide slope intercept altitude is simply the altitude where you meet the glide slope and begin tracking it down.
Why Pilots Care
Descending below this altitude before intercepting the glide slope can put the aircraft below obstacle-clearance protection. Holding it correctly ensures a safe, stabilized capture of the glide slope and a normal descent to the runway.
Grounding Statement
Picture flying level toward the runway until the glide slope reaches you; the altitude you should be holding at that moment is the GS intercept altitude.
Intuition Check
Intercept does not mean to block or stop something here. It means to meet the glide slope and begin following it at the published altitude.
Example Sentence 1
The crew leveled off at the GS intercept altitude of 2,000 feet and waited for the glide slope indicator to come alive before starting the descent.
Example Sentence 2
We leveled at the GS intercept altitude and waited for the glide slope to center before descending.