Other forms: brightened; brightening; brightens
To brighten is to make something lighter, the way you brighten your back yard with glowing patio lights or brighten up your gloomy bedroom by painting the walls yellow.
When the sun comes out after several rainy days, it brightens your house and the walk to school, but it can also brighten your mood. This verb is perfect for both ways of making things light and cheerful — adding something that emits light or lending a happy or hopeful mood to a situation. Your best friend may be so funny and positive that they brighten a room just by walking in.