A plant or animal species that is in danger of extinction because its population numbers have reached a critical level or its habitats have been drastically reduced. If these causal factors continue, the species is unlikely to survive. As defined by the IUCN (International Union for the Conservation of Nature and Natural Resources), a critically endangered species is at ‘extremely high risk of extinction in the wild’; an endangered species is at ‘high risk of extinction in the wild’; and a vulnerable species is one likely to become endangered unless measures are taken to improve its circumstances in the wild. These are all categories of threatened species, a list of which is published by the IUCN in its Red Book.

http://www.iucn.org/ Website of the IUCN