A mass extinction marks a time when the biodiversity of life on earth suddenly crashes. Species go extinct for any number of reasons, and so a normal background rate of extinction does exist, just as a normal mortality rate exists within a single species. Just as a sudden, sharp increase in that mortality rate occurs during a genocide, a mass extinction points to an analogous atrocity but increased to the scale of all life on earth.
Life on earth has survived six mass extinctions, the most recent only ending with the dawn of the Fifth World. This most recent event, in the end, had the lowest toll — only about half of all species — but it unraveled much more quickly than any previous event, including the End Permian extinction which wiped out as much as 98% of all life on the planet at the time. The End Permian extinnction unfolded over many millions of years, though, whereas the most recent event wiped out half of all species in a scant 10,000 years.
This finally ended with the collapse of civilization and the change in global climate. Of course, on the timeline of mass extinctions this amount of time seems hardly noticeable at all, so the impact of this mass extinction forms one of the most important facts of life in the Fifth World.