What is meant by lenticular cloud?

A lens-shaped cloud, seen over mountains or hills. It has sharp edges, and the numerous cloud jets of which it is formed are generally seen streaming through the cloud. It is associated with a Fohn Wind.

There are three types of lenticular clouds: altocumulus standing lenticular (ACSL), stratocumulus standing lenticular (SCSL), and cirrocumulus standing lenticular (CCSL). Because of their shape, they have been offered as an explanation for some unidentified flying object (UFO) sightings.