1] Virtual height : It is the height of the wave measured from earth's surface to that point where the refracted wave could have reflected or seems to have been reflected from.
2] MUF : Maximum usable frequency is the highest possible frequency that can be reflected back from ionosphere at a critical angle to earth's surface.
3] Skip distance D : Skip distance is the distance from the transmitter to the point on earth where the sky wave first returns to the earth.
4] Skip zone : The skip zone is a zone of silence between the point where the sky wave is first returned to earth, i.e. between transmitter & receiver for skip distance.