Nomen oblitum
A nomen oblitum (Latin for "forgotten name") is a name that has not been used in the scientific community for more than fifty years after its original proposal, and which is either a senior synonym or homonym; that is, a more recent name which is in common use is either the same taxon (a synonym), or is spelled the same (a homonym). Once formally declared as such, the senior is no longer recognized, as the junior name takes precedence, as a nomen conservandum or nomen protectum.
A notable example is that of Scrotum humanum.