A monocular is a modified refracting telescope used to magnify the images of distant objects by passing light through a series of lenses and sometimes prisms; the use of prisms results in a lightweight telescope. Volume and weight are less than half those of binoculars of similar optical properties, making them very easy to carry.

Monoculars produce 2-dimensional images, while binoculars add perception of depth (3 dimensions). A monocular with a straight optical path is relatively long; prisms can be used to fold the optical path to make an instrument which is much shorter.