Definition
The minimum altitude, published on an instrument approach chart, at which an aircraft must be established before intercepting the glideslope (for an ILS) or glidepath (for an LPV, LNAV/VNAV, or similar vertically guided approach) inbound to the runway. It is depicted on the profile view of the approach chart, typically by a lightning-bolt symbol at the final approach fix, and represents the altitude at which the descent path is joined for the final approach segment.
Plain English
The altitude you must be at -- and no lower than -- when you join the descending beam that guides you down to the runway on an instrument approach.
Context Anchor
Seen on instrument approach charts and during radar vectors to the final approach course, especially when ATC assigns an altitude before clearing the aircraft for the approach.
Derivation
Intercept' comes from the Latin intercipere, meaning 'to seize or catch between.' In aviation, you 'intercept' a beam or path by flying level until your aircraft crosses it -- catching it from below. The intercept altitude is simply the level you hold at while waiting to catch the descending path.
Why Pilots Care
Capturing the glideslope at the published altitude prevents false glideslope signals and ensures a stable descent rate.
Grounding Statement
Picture flying level at the assigned altitude until the vertical path comes down to you, then starting the descent along it.
Intuition Check
“Intercept” does not mean crossing through the path and continuing on. Here it means meeting the vertical path and then following it. “Altitude” is not just any height; it is the specific height assigned or published for joining the descent path.
Example Sentence 1
ATC cleared us for the ILS, and we leveled at 2,000 feet, the published glideslope intercept altitude, until the needle came alive.
Example Sentence 2
We reached the glideslope/glidepath intercept altitude and immediately captured the glidepath with a smooth transition to approach mode.