The Collaborative International Dictionary of English v.0.48:

Perturbation \Per`tur*ba"tion\, n. [L. perturbatio: cf. F.
   perturbation.]
   1. The act of perturbing, or the state of being perturbed;
      esp., agitation of mind.
      [1913 Webster]

   2. (Astron.) A disturbance in the regular elliptic or other
      motion of a heavenly body, produced by some force
      additional to that which causes its regular motion; as,
      the perturbations of the planets are caused by their
      attraction on each other. --Newcomb.
      [1913 Webster]

WordNet (r) 3.0 (2006):

perturbation
    n 1: an unhappy and worried mental state; "there was too much
         anger and disturbance"; "she didn't realize the upset she
         caused me" [syn: disturbance, perturbation, upset]
    2: (physics) a secondary influence on a system that causes it to
       deviate slightly
    3: activity that is a malfunction, intrusion, or interruption;
       "the term `distress' connotes some degree of perturbation and
       emotional upset"; "he looked around for the source of the
       disturbance"; "there was a disturbance of neural function"
       [syn: perturbation, disturbance]
    4: a disposition that is confused or nervous and upset [syn:
       perturbation, fluster]
    5: the act of causing disorder [syn: disruption,
       perturbation]