Term used in the NatureServe CDC network to designate an area (point or map polygon) in which an element of biodiversity is, or was, present. An occurrence has a conservation value (rank) for this element. For species, the occurrence generally corresponds to the habitat occupied by a local population of the species in question. What constitutes an occurrence and the criteria used to assign the related element occurrence rank varies depending on the element of biodiversity under consideration. The occurrence may correspond to a single map polygon (line or observation point) or to a group of nearby polygons.