per·tur·ba·tion 
(pûr′tər-bāshən)
(pûr′tər-bāshən)n.
1.
a. The act of perturbing.
b. The state of being perturbed; agitation.
2. A small change in a physical system, such as a variation in a planet's orbit resulting from the gravitational influence of other celestial bodies.
per′tur·bation·al adj.