(abbreviation n. o., plural nomina oblita) A nomen oblitum (Latin, forgotten name) is one that is technically a senior synonym of another, more recent name, but which has been used little or not at all since its original publication, and which would cause confusion if resurrected. Under the International Code of Zoological Nomenclature, to qualify as a nomen oblitum a name must not have been used as valid since 1899, and the competing junior name must have appeared in at least 25 works by at least 10 authors in the immediately preceeding 50 years and over a period not exceeding 10 years. The term "nomen oblitum" has also been used in the past for names suppressed by the International Commission on Zoological Nomenclature. A name that remains in place due to its senior synonym being a nomen oblitum is called a nomen protectum. (see ICZN online for more details)
For example, the name Tyrannosaurus rex Osborn, 1905 is a junior synonym of Manospondylus gigas Cope, 1892. However, because of the obscurity of the name Manospondylus compared to the name Tyrannosaurus, the former has been declared a nomen oblitum, and Tyrannosaurus rex remains the correct name.
Unlike the ICZN, the International Code of Botanical Nomenclature does not have any provisions for automatic rejection of an old name, requiring an action by the Commission for any name suppression. It is therefore not uncommon in botanical nomenclature for old names to be resurrected.