Plain English Meaning
The point in space where a GPS approach officially begins. When the aircraft reaches this point, it stops being on the way to the airport and starts flying the approach.
Full Definition
The waypoint that marks the beginning of the initial approach segment on a GPS instrument approach procedure. It is the first fix at which an aircraft transitions from the en route or arrival phase into the approach itself, and from which the published initial approach course, altitude, and any required course reversal or procedure turn are flown.