A monocle is a single lens worn in front of one eye, held in place by the muscles around the eye socket. Unlike eyeglasses, which rest on your nose and have two lenses connected by a frame, a monocle is just one circular piece of glass that you press into your eye socket to keep it from falling out.

Monocles were popular in the 1800s and early 1900s, especially among wealthy European gentlemen who needed help seeing but thought wearing full eyeglasses looked undignified. A monocle dangled from a chain or ribbon attached to the wearer's jacket, so when it popped out (which could happen), it wouldn't drop to the ground. Picture a stern-looking Victorian gentleman peering down at you through a single lens, and you've got the image.

Today, monocles are rarely used for actual vision correction. Modern eyeglasses and contact lenses work much better and don't require you to scrunch up your face to keep them in place. But monocles still appear in movies, books, and costumes to suggest old-fashioned wealth or sophistication. Think of a character in a top hat peering through a single lens at a fancy dinner party.