International Union for Conservation of Nature - Species Designations
The IUCN is the best known worldwide conservation status listing and ranking system. Species are classified by the IUCN Red List into nine groups set through criteria such as rate of decline, population size, area of geographic distribution, and degree of population and distribution fragmentation.
Also included are species that have gone extinct since 500 CE. When discussing the IUCN Red List, the official term "threatened" is a grouping of three categories: critically endangered, endangered, and vulnerable.
Extinct (EX) – No known living individuals
Extinct in the wild (EW) – Known only to survive in captivity, or as a naturalized population outside its historic range
Critically endangered (CR) – Extremely high risk of extinction in the wild
Endangered (EN) – High risk of extinction in the wild
Vulnerable (VU) – High risk of endangerment in the wild
Near threatened (NT) – Likely to become endangered in the near future
Least concern (LC) – Lowest risk; does not qualify for a higher risk category. Widespread and abundant taxa are included in this category.
Data deficient (DD) – Not enough data to make an assessment of its risk of extinction
Not evaluated (NE) – Has not yet been evaluated against the criteria.