A monocle is a single lens that people wear over one eye for fashion or as part of a costume.
You might have seen a picture of a stuffy old aristocrat from a historical movie or book, with a monocle perched on his nose. That's not so different from how the word "monocle" is used today in popular culture - as a symbol of elegance and sophistication. Some people wear single lenses for fashion, and they're often called "sunglasses," but if you want to sound fancy, you can call them monocles.
A single lens, usually in a wire frame, and used to correct vision for only one eye.
