monocular
A monocular is a modified refracting telescope used to magnify images of distant objects by passing light through a series of lenses, usually prisms, the application of which results in a lightweight and compact telescope. The typical size and weight of a monocular is less than half that of a pair of binoculars with similar optical properties, making monoculars more portable and correspondingly less expensive.
Monoculars are ideal for those who have vision in only one eye, or for places where compactness and low weight are important, such as hiking. Sometimes monoculars are also preferred when using both eyes through binoculars becomes difficult due to large eye changes or poor vision in one eye.