Q.Define skip distance.

When a signal is beamed into the ionosphere, only rays launched within the critical angle are returned to earth, and they do not come back down right next to the transmitter — they land some distance away. This gives rise to two closely linked ideas.

The skip distance is the shortest (minimum) distance, measured along the earth's surface, between the transmitting antenna and the first point at which the returning sky wave reaches the ground. No sky-wave signal can be received closer to the transmitter than this. The skip distance depends on the frequency used and on the state of the ionosphere. …