An undertone is a subtle quality, feeling, or meaning that lies beneath the surface of what's being said or done. When someone speaks with an undertone of sadness, you can hear it in their voice even if they're not directly saying they feel sad. The actual words might be cheerful, but something in how they speak reveals the hidden emotion.

In conversations, undertones matter enormously. A teacher might ask “Did you finish your homework?” in a neutral tone, but an undertone of disappointment tells you she already suspects you didn't. Your friend might say “That's fine” about changing plans, but an undertone of annoyance reveals they're actually bothered.

The word can also describe literal quieter sounds beneath louder ones. When an orchestra plays, you might notice undertones from the cellos supporting the bright melody from the violins.

People often miss undertones because they focus only on the obvious, surface-level meaning. Learning to notice undertones helps you understand what people really mean and feel. When someone says your science project is “interesting” with an undertone of genuine curiosity, that's very different from saying it with an undertone of confusion or doubt.