Sober means serious, calm, and thoughtful rather than silly or excitable. After a fire drill, a teacher might use a sober tone to discuss fire safety, showing that the topic deserves careful attention. A student might give a sobering presentation about endangered species, meaning it makes the audience think seriously about an important problem.

You might also encounter sober describing plain, understated things. Sober colors are quiet and neutral, like gray or navy blue, rather than bright or flashy. A room with sober decorations feels calm and dignified rather than exciting or busy.

Note: Sober can also mean not drunk or affected by alcohol or drugs.