Unique-event polymorphism

In genetic genealogy, a unique-event polymorphism (UEP) is a genetic marker that corresponds to a mutation that is likely to occur so infrequently that it is believed overwhelmingly probable that all the individuals who share the marker, worldwide, will have inherited it from the same common ancestor, and the same single mutation event. Generally, UEP is an allele for which all copies derive from a single mutational event.

Source: Wikipedia — Unique-event polymorphism (CC BY-SA 4.0)