perturbation

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The act of perturbing.
• noun The state of being perturbed; agitation.
• noun 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.

from The Century Dictionary.

• noun The act of perturbing, or the state of being perturbed; disturbance; disorder; especially, disquiet of mind; restlessness or want of tranquillity of mind; commotion of the passions.
• noun Variation; especially, irregular or violent variation.
• noun A cause of disquiet.
• noun In astronomy, a deviation of the motion of a planet or comet from a fixed orbit or from its regular velocity in that orbit.

from the GNU version of the Collaborative International Dictionary of English.

• noun The act of perturbing, or the state of being perturbed; esp., agitation of mind.
• noun (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun uncountable Agitation; the state of being perturbed
• noun countable A small change in a physical system, or more broadly any definable system (such as a biological or economic system)
• noun countable, astronomy, physics Variation in an orbit due to the influence of external bodies

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a disposition that is confused or nervous and upset
• noun (physics) a secondary influence on a system that causes it to deviate slightly
• noun activity that is a malfunction, intrusion, or interruption
• noun the act of causing disorder
• noun an unhappy and worried mental state