Nomen illegitimum

Nomen illegitimum (Latin for illegitimate name) is a technical term, used mainly in botany. It is usually abbreviated as nom. illeg. Although the International Code of Nomenclature for algae, fungi, and plants uses Latin terms for other kinds of name (e.g. nomen conservandum for "conserved name"), the glossary defines the English phrase "illegitimate name" rather than the Latin equivalent. However, the Latin abbreviation is widely used by botanists and mycologists.

A superfluous name is often an illegitimate name. Again, although the glossary defines the English phrase, the Latin equivalent nomen superfluum, abbreviated nom. superfl. is widely used by botanists.

A nomen illegitimum is a validly published name, but one that contravenes some of the articles laid down by the International Botanical Congress. The name could be illegitimate because:

For the procedure of rejecting otherwise legitimate names, see conserved name.

The qualification above concerning the taxon and the type is important. A name can be superfluous but not illegitimate if it would be legitimate for a different circumscription. For example, the family name Salicaceae, based on the "type genus" Salix, was published by Charles-François Brisseau de Mirbel in 1815. So when in 1818 Lorenz Chrysanth von Vest published the name Carpinaceae (based on the genus Carpinus) for a family explicitly including the genus Salix, it was superfluous: "Salicaceae" was already the correct name for Vest's circumscription; "Carpinaceae" is superfluous for a family containing Salix. However, the name is not illegitimate, since Carpinus is a legitimate name. If Carpinus were in future placed in a family where no genus had been used as the basis for a family name earlier than Vest's name (e.g. if it were placed in a family of its own) then Carpinaceae would be its legitimate name. (See Article 52.3, Ex. 18.)

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