A technology that by default destroys the civilization that invents it. It's one type of Existential Risk. AGI may be such an invention.
The name comes from a thought experiment by Nick Bostrom, where he described inventions as pulling marbles out of an urn. Most are white (beneficial), some are dangerous or a mixed blessing (usually described as gray or red balls), and some are black (fatal).
Also sometimes phrased as "black ball" when describing the experiment, including by Bostrom himself, but that already means something else when used by itself.