To lighten means to make something less heavy or less dark. When you lighten your backpack by removing books you don't need for homework, you make it easier to carry. When clouds lighten from dark gray to white, they become brighter and the day feels more cheerful.

The word also describes making a situation less serious or tense. When a teacher lightens the mood with a joke during a difficult lesson, everyone relaxes a bit. A funny story can lighten an awkward moment at a family dinner. You might try to lighten a friend's worries by reminding them of something positive.

Lighten up means to stop being so serious or worried. If someone tells you to lighten up, they think you're taking something too seriously or being too hard on yourself or others.

The word can also mean to make colors paler. Hair salons lighten people's hair color, and painters lighten paint by adding white.