Atavism
This is a biological feature that reappears after being absent for sometime – usually many generations.
For example, every once in a while, a human is born with a tail stump, or covered in fur. Additionally, very rarely, someone will find a whale with legs.
Socially, an atavism can refer to something which is “throwback” or “retro.” When bell-bottoms come back into style, that can be considered a fashion atavism.
Why I Looked It Up
In Neuromancer:
You couldn’t kill a zaibatsu by assassinating a bunch of executives; there were others waiting to step up the ladder to the vacated position. […] T-A was an atavism, a clan.
The usage here is odd. The speaker is comparing a large corporation (a “zaibatsu”) as a sort of immortal being, like genetic traits that you think are gone but eventually re-surface.
Update
Added on
From an article about social media:
I don’t know what other word describes the feeling I get when, every day I log on to Twitter (now known as X) or Instagram or Substack or whatever and witness an atavism that is ugly, cynical, self-serving, and varying degrees of brutal.