Source : Webster's Revised Unabridged Dictionary (1913)

Atavism \At"a*vism\, n. [L. atavus an ancestor, fr. avus a
   grandfather.]
   (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.

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

Source : WordNet®

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