Plain English Meaning
The moment the aircraft stops flying level and starts following the descent path down to the runway.
Full Definition
The point during an instrument approach at which the aircraft transitions from level flight to descending along the published or radar-provided glidepath. On a precision approach, this is where the aircraft meets the descending guidance signal (such as the ILS glideslope or PAR glidepath) and begins its controlled descent toward the runway.
Context Anchor
Seen during instrument approaches, especially radar approaches, when a controller or indication helps join the proper descent path.