Webster's English Dictionary

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perturbation

per.tur.ba.tion \.p*rt-*r-'ba--sh*n, .p*r-.t*r-\ \-shn*l, -sh*n-*l\ n 1: 
   the action of perturbing : the state of being perturbed  2: a disturbance 
   of the regular elliptic or other motion of a celestial bod y produced by 
   some force additional to that which causes its regular motion - 
   per.tur.ba.tion.al aj