1 the act or an instance of perturbing; the state of being perturbed.
2 a cause of disturbance or agitation.
3 Physics a slight alteration of a physical system, e.g. of the electrons in an atom, caused by a secondary influence.
4 Astron. a minor deviation in the course of a celestial body, caused by the attraction of a neighbouring body.
Useful english dictionary. 2012.