Nomen oblitum

In zoological nomenclature, a nomen oblitum (plural: nomina oblita; Latin for "forgotten name") is a disused scientific name which has been declared to be obsolete (figuratively "forgotten") in favor of another "protected" name.

In its present meaning, the nomen oblitum came into being with the fourth edition (1999) of the International Code of Zoological Nomenclature. After 1 January 2000, a scientific name may be formally declared to be a nomen oblitum when it satisfy the following conditions:

1. No uses in a scientific publication are known after 1899 (this criterion is taken on faith).
2. It is either a senior synonym (there is also a more recent name which applies to the same taxon, and which is in common use) or a senior homonym (it is spelled the same as another name, which is also in valid use).
3. The preferred junior synonym or homonym is shown to be in wide use, defined as appearing in 25 or more publications in the past 50 years (must cover a period of at least 10) and by at least 10 distinct individual authors.

Once a name has formally been declared to be a nomen oblitum, the now obsolete name is to be "forgotten". By the same act, the other available name must be declared to be protected under the title nomen protectum. Thereafter it takes precedence. This procedure as a whole is termed a reversal of precedence.^[1] An example is the case of the scientific name for the leopard shark. Despite the name Mustelus felis being the senior synonym, an error in recording the dates of publication resulted in the widespread use of Triakis semifasciata as the leopard shark's scientific name. After this long-standing error was discovered, T. semifasciata was made the valid name (as a nomen protectum) and Mustelus felis was declared invalid (as a nomen oblitum).^[2]