Nomen nudum
In taxonomy, a nomen nudum ('naked name'; plural nomina nuda) is a designation which looks exactly like a scientific name of an organism, and may have originally been intended to be one, but it has not been published with an adequate description. This makes it a "bare" or "naked" name, which cannot be accepted as it stands. A largely equivalent but much less frequently used term is nomen tantum ("name only"). Sometimes, "nomina nuda" is erroneously considered a synonym for the term "unavailable names". However, not all unavailable names are nomina nuda.
In zoology
According to the rules of zoological nomenclature a nomen nudum is unavailable; the glossary of the International Code of Zoological Nomenclature gives this definition:
nomen nudum (pl. nomina nuda), n.
A Latin term referring to a name that, if published before 1931, fails to conform to Article 12; or, if published after 1930, fails to conform to Article 13. […]
And among the rules of that same Zoological Code:
12.1. To be available, every new name published before 1931 must … be accompanied by a description or a definition of the taxon that it denotes, or by an indication [that is, a reference to such a description or definition]. …
13.1. To be available, every new name published after 1930 must … be accompanied by a description or definition that states in words characters that are purported to differentiate the taxon, or be accompanied by a bibliographic reference to such a published statement.
In botany
According to the rules of botanical nomenclature a nomen nudum is not validly published. The glossary of the International Code of Nomenclature for algae, fungi, and plants gives this definition:
A designation of a new taxon published without a description or diagnosis or reference to a description or diagnosis.
The requirements for the diagnosis or description are covered by articles 32, 36, 41, 42, and 44.
Nomina nuda that were published before 1 January 1959 can be used to establish a cultivar name. For example, Veronica sutherlandii, a nomen nudum, has been used as the basis for Hebe pinguifolia 'Sutherlandii'.