Definition of "perturbation"

perturbation

noun
countable and uncountable, plural perturbations
1. (uncountable) Agitation; the state of being perturbed
  Quotations
  But her mind had never been in such perturbation; and it needed a very strong effort to appear attentive and cheerful till the usual hour of separating allowed her the relief of quiet reflection.
  1815, Jane Austen, Emma, volume I, chapter 5
2. (countable) A small change in a physical system, or more broadly any definable system (such as a biological or economic system)
  Quotations
  Natures that haue much Heat, and great and violent deſires and Perturbations, are not ripe for Action, till they haue paſſed the Meridian of their yeares: As it was with Iulius Cæſar, and Septimius Seuerus.
  1625, Francis [Bacon], “Of Youth and Age. XLII.”, in The Essayes […], 3rd edition, London: […] Iohn Haviland for Hanna Barret, pages 247–248
3. (countable, astronomy, physics) Variation in an orbit due to the influence of external bodies