A single lens worn in front of one eye to help you see better, or a monocle; can also mean eyeglasses (spectacles).
From 'eye' + 'glass' (Old English glæs). Single eyeglasses for one eye were common before modern spectacles; the term evolved to refer to any corrective lens.
The monocle—a single eyeglass held by the eye muscle—was a status symbol for wealthy Europeans in the 1800s, even though it was awkward and fell out constantly because physics doesn't care about fashion.
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