返祖 (fǎn zǔ) literally means 'returning to the ancestor' — '返' means 'to return' or 'to revert', and '祖' means 'ancestor' or 'origin'. Together, they describe the biological phenomenon of atavism: when an organism displays traits that were present in distant ancestors but had disappeared in recent generations, such as extra toes in horses or vestigial tails in humans. This term is used primarily in biology, genetics, and evolutionary science.

While rooted in scientific discourse, 返祖 occasionally appears metaphorically in informal contexts — for example, describing a person’s sudden reversion to childhood habits or outdated social behaviors — though this usage is less common and often carries a mildly critical or humorous tone. It is not used for everyday regression (e.g., feeling tired); instead, it implies a striking, biologically or culturally ancestral echo. The word functions strictly as a noun and does not take aspect markers or verb complements.