Endangered Species , Definition

The IUCN (the World Conservation Union) is the organization that WWF and other conservation groups, government agencies, scientists and academics look to for that information. IUCN brings together the world's leading scientists, including those from WWF, to assess the conservation status of species, subspecies, varieties and subpopulations on a global scale, highlighting species threatened with extinction and promoting their conservation.
IUCN maintains a complete list of all the species it considers critically endangered, endangered or vulnerable: It's called the Red List of Threatened Species.