The Collaborative International Dictionary of English v.0.48:

Atavism \At"a*vism\, n. [L. atavus an ancestor, fr. avus a
   grandfather.]
   1.
      (a) The recurrence, or a tendency to a recurrence, of the
          original type of a species in the progeny of its
          varieties; resemblance to remote rather than to near
          ancestors; reversion to the original form.
      (b) (Biol.) The recurrence of any peculiarity or disease
          of an ancestor in a subsequent generation, after an
          intermission for a generation or two.
          [1913 Webster]

                Now and then there occur cases of what
                physiologists call atavism, or reversion to an
                ancestral type of character.      --J. Fiske
          [1913 Webster]

   2. recurrence of or reversion to a past style, outlook,
      approach, or manner.
      [PJC]

WordNet (r) 3.0 (2006):

atavism
    n 1: a reappearance of an earlier characteristic [syn:
         atavism, reversion, throwback]