A mass extinction is an event in which a significant percentage of all life on Earth becomes extinct over a relatively short geological time period. Mass extinctions are critical for understanding the history of life on Earth, the factors that can lead to widespread species loss, and the subsequent recovery and evolution of ecosystems.
Reference: Raup, D. M., & Sepkoski, J. J. (1982). “Mass Extinctions in the Marine Fossil Record.” Science, 215(4539), 1501-1503.