Skip distance is the:

Skip distance is officially defined as the minimum distance at which a radio wave returns to Earth after being refracted (or reflected) by the ionosphere. It marks the starting point where you can hear a sky-wave signal from a distant transmitter. The space between the furthest reach of the transmitter's ground wave and this minimum sky-wave distance is known as the "skip zone," a dead zone where the signal cannot be heard. Distractors mentioning "maximum distance" are incorrect because skip distance strictly refers to the closest point the ionospheric reflection lands back on the surface, not the furthest limit of the signal's travel.