In Ballet, the rond de jambe (from the French; a circling of the leg) comes in two forms: the rond de jambe à terre or en l'air.
The first variation means 'a circling of the leg on the ground', and the movement described is just that. The leg is extended to the front (en avant), then moved round in a smooth oval to the side (en face) and on to the back (en arrière). Care must be taken at all times to ensure the leg is properly stretched, and that it is rotating correctly in the hip socket with no twisting of the limb.
'En l'air' translates as 'in the air', and the movement of the leg is similar to the rond de jambe à terre, but in this case the limb is raised, ideally at right angles to the body. The same considerations of style as for the previous example should be kept in mind; perhaps more so, as the exercise is more difficult to do well.
An exercise in rond de jambe helps the knee joints to become more flexible and builds strength in the legs. Later on, fondu or bending movements can be added to the exercise to test flexibility and endurance.