Opossums (colloquially possums)(Didelphimorphia, /daɪˌdɛlfɨˈmɔrfiə/) make up the largest order of marsupials in the Western Hemisphere, including 103 or more species in 19 genera. They are also commonly called possums, though that term technically refers to Australian fauna of the suborder Phalangeriformes. The Virginia opossum was the first animal to be named an opossum; usage of the name was published in 1610. The word opossum was borrowed from the Virginia Algonquian (Powhattan) language in the form aposoum and ultimately derives from the Proto-Algonquian word *wa˙p- aʔθemw, meaning "white dog" or "white beast/ animal". Opossums probably diverged from the basic South American marsupials in the late Cretaceous or early Paleocene.

Their unspecialized biology, flexible diet and reproductive strategy make them successful colonizers and survivors in diverse locations and conditions.^{[citation needed]}

Didelphimorphs are small to medium-sized marsupials, with the largest just exceeding the size of a large house cat, and the smallest the size of a small mouse. They tend to be semi-arboreal omnivores, although there are many exceptions. Most members of this taxon have long snouts, a narrow braincase, and a prominent sagittal crest. The dental formula is: Failed to parse (Missing texvc executable; please see math/README to configure.): \tfrac{ 5.1.3.4}{ 4.1.3.4} . By mammalian standards, this is a very full jaw. The incisors are very small, the canines large, and the molars are tricuspid.