Perturbation:
(noun)
1. a disturbance of motion, course, arrangement, or state of equilibrium; especially: a disturbance of the regular and usually elliptical course of motion of a celestial body that is produced by some force additional to that which causes its regular motion
2. the origin of a phase transition, or topological defect, created by force resulting in material (structural) failure
3. anxiety; mental uneasiness
