1. A small change in a physical system: A perturbation most often takes place when the equilibrium is disturbed from the outside.
2. A cause of mental disquiet, disturbance, or agitation: Tim's bizarre haircut was the perturbation that upset his parents at Easter dinner.
3. Variation in a designated orbit, as of a planet, that results from the influence of one or more external bodies: Gravitational attraction between planets can cause perturbations and cause a planet to deviate from its expected orbit.

Perturbations in Neptune's orbit led to the discovery of the object that was causing the perturbation, the planet Pluto.

Perturbations in the orbits of stars have led to the discovery of planetary systems outside of our Solar system.