perturbation

noun

• 1 [mass noun] anxiety; mental uneasiness:she sensed her friend’s perturbation
• [count noun] a cause of anxiety or uneasiness:Frank’s atheism was more than a perturbation to Michael

• 2a deviation of a system, moving object, or process from its regular or normal state or path, caused by an outside influence: these shifts and swings in wildlife populations are possibly related to climatic perturbations
• Astronomy a minor deviation in the course of a celestial body, caused by the attraction of a neighbouring body.