Other forms: monocles
While eyeglasses have two round lenses, one for each eye, a monocle has only one, and it's worn over a single eye.
It is rare to see anyone wearing a monocle these days, in part because it is held on the face solely by tensing the muscles around the eye over which it's worn. It was a popular part of a man's stylish ensemble beginning in the late 1700's, a fashion that lasted almost 100 years. The word monocle means "one-eyed" in French, from the Greek root monos, "single," and the Latin oculus, or "eye."