A smooth form of soarable lift found in the lee of steep mountain ranges. It requires that the wind be close to perpendicular to the range and increase in strength with altitude but can permit soaring to very high altitudes: recent world record of 48,000' with attempts soon (2002) to be made for 62,000' and eventually 100,000'.

Multiple rotors (rotating air) form behind and a little above the top of the mountain range, perhaps between 10,000' and 25,000'. Gliders entering strong rotor are liable to be pulled apart. The rotors and lenticular clouds formations above them are stationary with respect to the ground, the wind blows through them. The high-speed air above the rotors oscillates up and down over the rotors, forming lenticular clouds at each peak of the oscillation.

These lenticular clouds are what glider pilots like to see - they indicate strong, smooth lift that can reach to extremely high altitudes - great for attaining that diamond altitude badge.

A proper explanation & understanding of wave lift and the oscillations involved requires a diagram that is not conducive to an ascii art rendering. I got my description from Basic Gliding Knowledge, 4th edition 1996, published by the Gliding Federation of Australia. A google search for "wave lift" will give you instant gratification though :)