Dim means not very bright or having little light. A dim room has some light but not enough to see clearly, like a bedroom lit only by a nightstand lamp or a hallway with a single bulb at the far end. When you dim the lights in a movie theater, you're making them less bright so people can focus on the screen.

Dim can also describe something that's hard to see clearly because it lacks brightness or sharpness. You might have a dim memory of something that happened when you were very young: you remember it was there, but the details are fuzzy and unclear, like trying to see through fog. A dim outline of mountains appears faintly on the distant horizon.

When people say someone is dim, they mean the person isn't very smart or quick to understand things. This usage is unkind, so it's better to avoid it. You might also hear someone say the prospects look dim, meaning the chances of success seem small or unlikely, like clouds blocking out the sun and making everything darker.

The opposite of dim is bright: bright lights, bright memories, or bright prospects all suggest clarity, strength, and visibility.