Endemism is the ecological state of being unique to a place. Endemic species are not naturally found elsewhere. The place must be a discrete geographical unit, such as an island, habitat type, or other defined area or zone. For example, the Orange-breasted Sunbird is endemic to Fynbos, meaning it is exclusively found in the Fynbos vegetation type of southwestern South Africa.

There are two subcategories of endemism - paleoendemism and neoendemism. Paleoendemism refers to a species that was formerly widespread but is now restricted to a smaller area. Neoendemism refers to a species that has recently arisen such as a species that has hybridized and is now classified as a separate species. This is a common process in plants especially those which exhibit polyploidy.

An opposite notion is cosmopolitan distribution.

Endemic types or species are especially likely to develop on islands because of their geographical isolation. This includes remote island groups, such as Hawaii, the Galápagos Islands and Socotra. Endemism can also occur in biologically isolated areas such as the highlands of Ethiopia, or large bodies of water like Lake Baikal.

Endemics can easily become endangered or extinct because of their restricted habitat and vulnerability to the actions of man, including the introduction of new organisms. There were millions of both petrels and "cedars" (actually junipers) in Bermuda when it was settled at the start of the 17th century. By the end of the century, the petrels were thought to be extinct, and cedars were driven nearly to extinction. This was caused by centuries of shipbuilding as well as the introduction of a parasite. Both petrels and cedars are very rare today, as are other species endemic or native to Bermuda.

Endemic organisms are not the same as indigenous organisms — a species that is indigenous to somewhere may be native to other locations as well. An introduced species, also known as a naturalized or exotic species, is an organism that is not indigenous to a given place or area.