The phrase nomen nudum (plural nomina nuda) is a Latin term, meaning "naked name", used in taxonomy (especially in zoological and botanical nomenclature). It may or may not be written in italics, depending on style.
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The term is used to indicate a designation which looks exactly like a scientific name of an organism, and may have originally been intended to be a scientific name, but fails to be one because it has not (or has not yet) been published with an adequate description (or a reference to such a description), and thus is a "bare" or "naked" name, one which cannot be accepted as it currently stands.
Because a nomen nudum fails to qualify as a formal scientific name, a later author can publish a real scientific name that is identical in spelling. If one and the same author puts a name in print, first as a nomen nudum and later publishes the name accompanied by a description that meets the formal requirements, then the date of publication of the latter, formally correct, publication becomes the name's date of establishment.
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'.