Nomen oblitum is Latin for 'forgotten name'. While this could theoretically be used in any number of interesting contexts, it is most often used as a technical term in biological nomenclature to refer to a taxonomic name that has not been used for a long time, and has has been commonly replaced in newer publications.
According to the International Code of Zoological Nomenclature, in order for nomen oblitum to be declared obsolete, these three things must all be true:
- The senior synonym or homonym has not been used as a valid name after 1899
- The junior synonym or homonym has been used for a particular taxon, as its presumed valid name, in at least 25 works, published by at least 10 authors in the immediately preceding 50 years.
- A paper must be published citing evidence for #2, and citing both names together, declare that the junior synonym or homonym is being made a nomen protectum ("protected name") in accordance with ICZN article 23.9.
This is not a process undertaken lightly or frequently. Generally, the older name has priority over the newer. Declaring a nomen oblitum takes an action by the International Commission on Zoological Nomenclature (ICZN) to formally recognize the change.
The most famous case of declaring nomen oblitum was the official 'forgetting' of the genus Manospondylus in favor of the more popular Tyrannosaurus. Tyrannosaurus rex Osborn, 1905 is a junior name to Manospondylus gigas Cope, 1892, and should by rights be declared a nomen superfluum (a superfluous name). However, the Manospondylus was based on a few fragmentary types, and was not well enough defined to gain a real foothold in the literature (technically, this is known as nomen dubium, a 'dubious name'). It should be noted that while this change is generally recognized as fact, as far as I can find it has not yet been formally declared by the ICZN.
Another famous example is the genus Megalosaurus, which was originally described by Richard Brookes in 1763. He called it Scrotum humanum, because the available fragment of femur had rounded bulbs on the end that he thought looked like a human scrotum. (A picture is available here.) A petition was submitted to the ICZN to have the name formally changed to the commonly used Megalosaurus; the ICZN rejected the petition on the basis that Scrotum humanum was never intended as a serious name, and that the fossils available at the time of this pseudo-naming were not enough to declare a formal name in any case. Technically, this would qualify Scrotum humanum as a nomina conservanda ('suppressed name') rather than a nomen oblitum, but this is splitting hairs even for the notoriously pedantic field of taxonomy.
Until 1973, the term nomen oblitum was not so rigorously defined; theoretically any previous designation of nomen oblitum would have to be reinspected under the new guidelines. Whether or not this will be done depends primarily on who cares about any given sub-sub-sub field of biology, and how pedantic they are feeling. There is not a systematic and directed search for out-of-date names, and many old labels fall into de facto oblitum without formal recognition.
Against all expectations, nomen oblitum has a fan page on Facebook.