A dimmer is a device that controls how bright a light is. Instead of just turning lights on or off, a dimmer lets you adjust them anywhere between completely dim and full brightness. You've probably seen dimmer switches that look like sliding bars or rotating knobs on walls, letting you dial the lights up for reading or down for watching a movie.

Dimmers work by controlling how much electricity flows to the light bulb. When you slide or turn the control, you're telling the dimmer to send more or less power, which makes the light brighter or darker. Theaters use dimmers to slowly fade the lights down before a show starts, creating that exciting moment when the audience knows the performance is about to begin.

The word can also describe something that gives off less light than something else. A candle is dimmer than a flashlight. A cloudy day is dimmer than a sunny one. When you say one thing is dimmer than another, you're comparing their brightness: the moon is much dimmer than the sun, even though it can still light your way on a dark night.