Threatened species are any species (including animals, plants, fungi, etc.) which are vulnerable to extinction in the near future. World Conservation Union (IUCN) is the foremost authority on threatened species, and treats threatened species not as a single category, but as a group of three categories: vulnerable, endangered, and critically endangered, depending on the degree to which they are threatened.
Species that are threatened are sometimes characterised by the population dynamics measure of critical depensation, a mathematical measure of biomass related to population growth rate. This quantitative metric is one method of evaluating the degree of endangerment.
Less-than-threatened categories are Near Threatened, Least Concern, and the no longer assigned category of Conservation Dependent. Species which have not been evaluated (NE), or do not have sufficient data (Data Deficient) also are not considered "threatened" by the IUCN.
Although threatened and vulnerable may be used interchangeably when discussing IUCN categories, the term threatened is generally used to refer to the three categories (critically endangered, endangered and vulnerable), while vulnerable is used to refer to the least at risk of those three categories. They may be used interchangeably in most contexts however, as all vulnerable species are threatened species (vulnerable is a category of threatened species); and, as the more at-risk categories of threatened species (namely endangered and critically endangered) must, by definition, also qualify as vulnerable species, all threatened species may also be considered vulnerable.
Subspecies, populations and stocks may also be classified as threatened.
Under the Endangered Species Act in the United States, "threatened" is the less protected the two protected categories. The Bay checkerspot butterfly (Euphydryas editha bayensis) is an example of a threatened subspecies protected under the ESA.