Undescribed taxon

In taxonomy, an undescribed taxon is a taxon (for example, a species) that has been discovered, but not yet formally described and named. The various Nomenclature Codes specify the requirements for a new taxon to be validly described and named. Until such a description has been published, the taxon has no formal or official name, although a temporary, informal name is often used. A published scientific name may not fulfil the requirements of the Codes for various reasons. For example, if the taxon was not adequately described, its name is called a nomen nudum. It is possible for a taxon to be "undescribed" for an extensive period of time, even if unofficial descriptions are published.

An undescribed species may be referred to with the genus name, followed by "sp.", but this abbreviation is also used to label specimens or images that are too incomplete to be identified at the species level. In some cases, there is more than one undescribed species in a genus. In this case, these are often referred to by a number or letter. In the shark genus Pristiophorus, for example, there were, for some time, four undescribed species, informally named Pristiophorus sp. A, B, C and D. (In 2008, sp. A was described as Pristiophorus peroniensis and sp. B as P. delicatus.) When a formal description for species C or D is published, its temporary name will be replaced with a proper binomial name.