Perturb means to disturb, worry, or unsettle someone. When something perturbs you, it bothers you enough to disrupt your peace of mind or throw off your concentration. A strange noise in the night might perturb you. An unexpected change in plans could perturb someone who likes routine.

The word carries a sense of mild to moderate disturbance rather than overwhelming panic. If your teacher seems perturbed by the class's behavior, she's troubled and a bit annoyed, but not furious. When scientists talk about something being perturbed, they mean it's been knocked slightly out of its normal state: a planet's orbit might be perturbed by a passing asteroid, or a careful experiment might be perturbed by an unexpected variable.

You might feel perturbed when something doesn't quite add up, when someone acts strangely, or when your usual plans get disrupted. It's that unsettled feeling that something's not right, even if you can't put your finger on exactly what's wrong. The related word perturbation describes either the act of disturbing something or the disturbed state itself.