n. 1. the presence of a genetic trait noted in a remote ancestor that did not continue to occur in more recent ancestors. Therefore, there was a reversion to an earlier type. 2. more generally, this is used to signify the reappearance of or reversion to an earlier primitive characteristic or form (e.g., behavioral atavism).
ATAVISM: “A person exhibiting atavism has regressed to an earlier stage of development.”