When you're on an airplane with your friend who's terrified of flying, you'll be able to sense her perturbation, even if she doesn't say a word about how scared she is. The noun perturbation means "anxiety or uneasiness."
Perturbation means "a state of agitation," or it can refer to whatever's causing the anxiety. Your brother's weird haircut could be the perturbation that's upsetting your mom at Thanksgiving dinner. It usually means a personal feeling of upset or nervousness, but perturbation can also be a physical disruption of some kind, like a perturbation in a river that makes the water become choppy and dangerous. In astronomy, perturbation is a change caused by the gravitational pull of a celestial body.