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]

Moby Thesaurus II by Grady Ward, 1.0:

66 Moby Thesaurus words for "atavism":
   Mnemosyne, aboriginality, affect memory, age, ancien regime,
   ancientness, anterograde memory, antiquity, backset,
   backward deviation, cobwebs of antiquity, collective memory,
   computer memory, disk memory, drum memory, dust of ages, eld,
   elderliness, eldership, emotional response, engram, falling back,
   great age, hoary age, hoary eld, information storage, inveteracy,
   kinesthetic memory, lapse, memory, memory bank, memory circuit,
   memory trace, mind, mneme, old age, old order, old style, oldness,
   primitiveness, primogeniture, primordialism, primordiality,
   race memory, recollection, recrudescence, recurrence, regression,
   relapse, remembrance, renewal, return, reversal, reverse,
   reversion, screen memory, senility, seniority, setback, skill,
   souvenir, tape memory, throwback, venerableness, verbal response,
   visual memory