The Collaborative International Dictionary of English v.0.48:

Vacillate \Vac"il*late\ (v[a^]s"[i^]l*l[=a]t), v. i. [imp. & p.
   p. Vacillated (v[a^]s"[i^]l*l[=a]`t[e^]d); p. pr. & vb. n.
   Vacillating.] [L. vacillare, vacillatum; cf. Skr. va[~n]c.]
   [1913 Webster]
   1. To move one way and the other; to reel or stagger; to
      waver.
      [1913 Webster]

            [A spheroid] is always liable to shift and
            vacillatefrom one axis to another.    --Paley.
      [1913 Webster]

   2. To fluctuate in mind or opinion; to be unsteady or
      inconstant; to waver.
      [1913 Webster]

   Syn: See Fluctuate.
        [1913 Webster]

WordNet (r) 3.0 (2006):

vacillate
    v 1: be undecided about something; waver between conflicting
         positions or courses of action; "He oscillates between
         accepting the new position and retirement" [syn: hover,
         vibrate, vacillate, oscillate]
    2: move or sway in a rising and falling or wavelike pattern;
       "the line on the monitor vacillated" [syn: fluctuate,
       vacillate, waver]