To brighten means to make something lighter, more cheerful, or more hopeful. When you turn on a lamp in a dim room, you brighten the space by adding light. When storm clouds move away and sunshine returns, the sky brightens.

The word works for moods and feelings too. A kind gesture can brighten someone's day, making them feel happier even if they were sad or frustrated before. When a teacher walks into class with exciting news about a field trip, faces brighten with smiles. A funny joke can brighten the mood during a long car ride.

You might brighten a room by painting the walls a lighter color or opening the curtains. The word suggests improvement and positive change, whether you're adding actual light or just making something feel more pleasant and optimistic.

Notice that brighten often implies a choice or change: you brighten something by doing something to it, whether that's flipping a switch, sharing good news, or smiling at someone who needs it.