(abbreviation n. d., plural nomina dubia) A nomen dubium (Latin, "doubtful name") is a taxon that has not been characterised in enough detail and whose type material is not sufficient for it to be identified. For instance, a number of dinosaur taxa named in the 1800s such as Trachodon were based on isolated teeth. Unfortunately, teeth in reptiles do not generally differ between species, meaning that fossilised teeth usually cannot be reliably identified to a particular species.
The significance of a taxon being declared a nomen dubium is often misunderstood. Contrary to popular belief, a nomen dubium is not invalid, in the way a nomen nudum is. A nomen dubium is still available for consideration in terms of synonymy and/or homonymy, and if a name previously regarded as a nomen dubium is able to identified with a better distinguished taxon that was named later, the nomen dubium is still the senior synonym, and hence the correct name for the taxon. One well-known example of this involves Allosaurus fragilis Marsh, 1877, which was suggested in the past as synonymous with Antrodemus valens Leidy, 1870, and Allosaurus appeared as Antrodemus in a number of older sources. However, Antrodemus is based on a single isolated tail bone, which is not sufficient to characterise the species. Allosaurus is currently regarded as a valid taxon, but this is because Antrodemus cannot be conclusively identified with it, not because Antrodemus is a nomen dubium. See New papers in Geobios (and nomenclatoral gripe) and follow-up messages on the Dinosaur Mailing List for an example of an argument on the appropriate application of a nomen dubium.