In scientific nomenclature, synonyms are different scientific names used for a single taxon. Usage and terminology are different for zoology and botany.

Zoology

In zoological nomenclature, synonyms are different scientific names that pertain to the same taxon, for example two names for the same species. The rule of zoological nomenclature is that the first name to be published is the senior synonym; any others are junior synonyms and should not be used.
Objective synonyms unambiguously refer to the same taxon; this is the case if they refer to the same description or the same type specimen. Otherwise the synonyms are subjective synonyms, meaning that there is room for debate: one researcher might consider the two (or more) types or descriptions to refer to one and the same taxon, another might consider them distinct.

For example, John Edward Gray published the name Antilocapra anteflexa in 1855 for a species of pronghorn, based on a pair of horns. However, it is now thought that his specimen was an unusual individual of the species Antilocapra americana published by George Ord in 1815. Ord's name thus takes priority, with Antilocapra anteflexa being a junior subjective synonym.

Objective synonyms are common at the level of genera, because two researchers may independently arrive at the conclusion that a species is sufficiently different from others in its genus that it needs to be given its own genus. Thus each names a new genus with the same type species; these are objective synonyms.

At the species level, subjective synonyms are common because an unexpectedly large range of variation in a species, or simple ignorance about an earlier description, may lead a biologist to describe a newly discovered specimen as a new species. However, objective synonyms are quite rare. An example is the tarpan (the European wild horse) which was described by Johann Friedrich Gmelin in 1774. In 1784 Pieter Boddaert named the tarpan Equus ferus, referring to Gmelin's description. Unaware of Boddaert's name, Otto Antonius published the name Equus gmelini in 1912, again referring to Gmelin's description. Since the two names refer to the same description, they are objective synonyms.

It is possible for a junior synonym to be given precedence over a senior synonym, primarily when the senior name has not been used since it was first described, and the junior name is in common use. The older name becomes a nomen oblitum, and the junior name is declared a nomen protectum. This is primarily to prevent the confusion that would result if a well-known name, with a large accompanying body of literature, were to be replaced by a completely unfamiliar name. For example, the scientific name of the Red imported fire ant, Solenopsis invicta, was published by Buren in 1972, and is a specific name that has been conserved, despite the fact that this species was first named Solenopsis saevissima wagneri by Santschi in 1916; there are thousands of publications that had been published using the name invicta before anyone discovered the synonymy, and, in 2001, the ICZN ruled that invicta would be given precedence over wagneri.

Botany

In botanical nomenclature, the synonym of a botanical name is a name that also applies to this same taxon. A synonym cannot exist in isolation: it is always "a synonym of ...". In botany synonyms can be:

In botany it is not required that a synonym be a valid name: a listing of synonyms often contains names that for some reason did not make it as a formal name (unpublished or manuscript names), or have not yet been formally published. Such a synonym must have the form of a formal name: it must look like a proper 'Latin name'.

A homotypic synonym need not share an epithet or name with the correct name, but only the type. For example the name Taraxacum officinale, mentioned above, has the same type as Leontodon taraxacum L. The latter is a homotypic synonym of Taraxacum officinale F.H.Wigg.

Comparison between the two

The treatment of synonyms in botanical nomenclature is quite different, in at least detail and terminology, from zoological nomenclature, where the correct name is included among synonyms, although as first among equals it is the "senior synonym":

• The synonyms in botany are "junior synonyms" in zoology.

• The homotypic or nomenclatural synonyms in botany are "objective synonyms" in zoology.

• The heterotypic or taxonomic synonyms in botany are "subjective synonyms" in zoology.

Other usage

The traditional concept of synonymy is often expanded in taxonomic databases to include pro parte (or "in part") synonyms. These are caused by splits and circumscriptional changes and do not require a one to one association with types. A single name could actually refer to several taxa because the original name is now used in a more restricted sense. They are usually indicated by the abbreviation p.p..