An atavism is a recurrence of or reversion to a past style, manner, outlook, approach, or activity. The word is derived from the Latin atavus. An atavus is a great-great-great-grandfather or, more generally, an ancestor.
In biology, an atavism is an evolutionary throwback, such as traits reappearing which had disappeared generations before. Atavisms occur because genes for previously existing phenotype features are often preserved in DNA, even though the genes are not expressed in some or most of the organisms possessing them.
In the social sciences, an atavism is a cultural tendency—for example, people in the modern era reverting to the ways of thinking and acting of a former time.