Holotype

A holotype is a single physical example (or illustration) of an organism that is the one that was used when the species (or lower-ranked taxon) was formally described. It may be the only such physical example (or illustration), or it may have been explicitly designated as the holotype from among several examples. Under the International Code of Zoological Nomenclature (ICZN), there are several kinds of name-bearing types, and a holotype is one of them. Between the International Code of Nomenclature for algae, fungi, and plants (ICN) and the ICZN, the definitions of types are similar in intent but not identical in terminology or underlying concept.

For example, the holotype for the butterfly Plebejus idas longinus is a preserved specimen of that subspecies, held by the Museum of Comparative Zoology at Harvard University. In botany and mycology, an isotype is a duplicate of the holotype, generally pieces from the same individual plant or samples from the same genetic individual.

A holotype is not necessarily "typical" of that taxon, although ideally it is. Sometimes just a fragment of an organism is the holotype, particularly in the case of a fossil. For example, the holotype of Pelorosaurus humerocristatus (Duriatitan), a large herbivorous dinosaur from the Early Cretaceous Epoch, is a fossil leg bone stored at the Natural History Museum in London. Even if a better specimen is subsequently found, the holotype is not superseded.