What does "atavism" mean?

Atavism: (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