In zoological nomenclature, a nomen dubium (Latin for "doubtful name", plural nomina dubia) is a scientific name that is of unknown or doubtful application. Note that in the ICBN and ICNB the phrase "nomen dubium" has no status.
In case of a nomen dubium it may be impossible to determine whether a specimen belongs to that group or not. This may happen if the original type specimen, or holotype, is lost or destroyed. All codes of nomenclature allow for a new type specimen, or neotype, to be chosen in this case.
A name may also become a nomen dubium if its holotype is fragmentary or lacking important diagnostic features. (This is often the case for species known only as fossils.) To preserve stability of names, the International Code of Zoological Nomenclature allows a new type specimen, or neotype, to be chosen for a nomen dubium in this case.
75.5. Replacement of unidentifiable name-bearing type by a neotype. When an author considers that the taxonomic identity of a nominal species-group taxon cannot be determined from its existing name-bearing type (i.e. its name is a nomen dubium), and stability or universality are threatened thereby, the author may request the Commission to set aside under its plenary power [Art. 81] the existing name-bearing type and designate a neotype.
For example, the crocodile-like archosaurian reptile Parasuchus hislopi Lydekker, 1885 was described based on a premaxillary rostrum (part of the snout), but this is no longer sufficient to distinguish Parasuchus from its close relatives. This made the name Parasuchus hislopi a nomen dubium. Texan paleontologist Sankar Chatterjee proposed that a new type specimen, a complete skeleton, be designated. The International Commission on Zoological Nomenclature considered the case and agreed in 2003 to replace the original type specimen with the proposed neotype.