In biological nomenclature, a nomen novum (Latin for "new name"), replacement name (or new replacement name, new substitute name, substitute name) is a replacement scientific name that is created when technical, nomenclatural reasons have made it impossible to continue using the previous name (for example because it was discovered to be a homonym – spelled the same as an existing, older name). Nomen novum does not apply when a name is changed for taxonomic reasons alone. It is frequently abbreviated, e.g. nomen nov., nom. nov..

In zoology establishing a new replacement name is a nomenclatural act and it must be expressly proposed to substitute a previously established and available name.

Often, the older name cannot be used because another animal was described earlier with exactly the same name. For example, Lindholm discovered in 1913 that a generic name Jelskia established by Bourguignat in 1877 for a European freshwater snail could not be used because another author Taczanowski had proposed the same name in 1871 for a spider. So Lindholm proposed a new replacement name Borysthenia. This is an objective synonym of Jelskia Bourguignat, 1877, because he has the same type species, and is used today as Borysthenia.

Also, for names of species new replacement names are often necessary. New replacement names have been proposed since more than 100 years ago. In 1859 Bourguignat saw that the name Bulimus cinereus Mortillet, 1851 for an Italian snail could not be used because Reeve had proposed exactly the same name in 1848 for a completely different Bolivian snail. Since it was understood even then that the older name always has priority, Bourguignat proposed a new replacement name Bulimus psarolenus, and also added a note why this was necessary. The Italian snail is known until today under the name Solatopupa psarolena (Bourguignat, 1859).

A new replacement name must obey certain rules; not all of these are well known.

Not every author who proposes a name for a species that already has another name, establishes a new replacement name. An author who writes "The name of the insect species with the green wings shall be named X, this is the one that the other author has named Y", does not establish a new replacement name (but a regular new name).

The International Code of Zoological Nomenclature prescribes that for a new replacement name, an expressed statement must be given by the author, which means an explicit statement concerning the process of replacing the previous name. It is not necessary to employ the term nomen novum, but something must be expressed concerning the act of substituting a name. Implicit evidence ("everybody knows why the author used that new name") is not allowed at this occasion. Many zoologists do not know that this expressed statement is necessary, and therefore a variety of names are regarded as having been established as new replacement names (often including names that were mentioned without any description, which is fundamentally contrary to the rules).

The author who proposes a new replacement name must state exactly which name shall be replaced. It is not possible to mention three available synonyms at once to be replaced. Usually, the author explains why the new replacement name is needed.