Plain English Meaning
The piece of sky set aside for you to turn around in. As long as you stay inside it while flying the course reversal, you are clear of obstacles and on a path the procedure was designed for.
Full Definition
The protected airspace surrounding a holding-pattern course-reversal procedure within which the aircraft must remain while completing the turn back onto the inbound course. It is the area the procedure designer has obstacle-evaluated for the maneuver, defined by the holding pattern's entry, turning, and outbound legs at the published altitude.
Context Anchor
Seen when studying or flying and other course reversals on an .