Coextinction of a species is the loss of one species upon the extinction of another. The term was originally used in the context of the extinction of parasitic insects following the loss of their hosts.

The term is now used describe the subsequent loss of any interacting species, including predators with their prey, and specialist herbivores with their food source. The most often cited example is that of the extinct passenger pigeon and its parasitic louse Columbicola extinctus .

In a recent article in Science, ecologist Lian Pin Koh and coworkers consider coextintion and state

Species coextinction is a manifestation of the interconnectedness of organisms in complex ecosystems. The loss of species through coextinction represents the loss of irreplaceable evolutionary and coevolutionary history. In view of the global extinction crisis, it is imperative that coextinction be the focus of future research to understand the intricate processes of species extinctions. While coextinction may not be the most important cause of species extinctions, it is certainly an insidious one.

(Science, Vol 305, Issue 5690, 1632-1634, 10 September 2004)

Koh also defines coendangered as "likely to go extinct if their currently endangered hosts ... become extinct"