In scientific classification, synonymy is the existence of multiple systematic names to label the same organism.

When an organism is described for the first time, the organism is, at a minimum, placed within a genus and a species and where applicable, it may be named as a subspecies, variety, subvariety or form of that species.

A name can be identified as the first name given or basionym by the author label. If the name used is the first one given to the organism, no brackets are used in the name, e.g. Acanthocalycium aurantiacum Rausch. If it is later decided that this name should change, the original author label is retained, but placed in parentheses, while the author(s) who defined the current name are added without parentheses.

It often happens that an organism that has already been described is later described again, and given a different name, by a scientist who was not aware of the earlier description, or did not recognise the organism as being the same as the one already described. In that case, when the identity of the two organisms is subsequently recognised, the earlier name generally takes priority, and the later name is noted as a synonym.

It can also happen that a renaming is proposed, and acquires some currency, but is not accepted; or it is accepted, but then abandoned in the light of later research, with a reversion to an earlier name. So the current name may not be the one most recently proposed.

For plant species, the IPNI database is a very extensive list of synonomies.

Some examples (basionym first, currently accepted name last):

• Acanthocalycium aurantiacum Rausch - 1968 (a popular small cactus)
  • Echinopsis aurantiaca (Rausch) Friedrich & G.D.Rowley - 1974
  • Lobivia thionantha var. aurantiaca (Rausch) Rausch - 1987

• Buxus chinensis Link 1836, the Jojoba
  • Simmondsia californica (Link) Nuttall
  • Simmondsia chinensis (Link) Schneider

• Butyrospermum parkii (G. Don) Kotschy, the Shea tree
  • Vitellaria paradoxa Gaertner f.
  • Butyrospermum paradoxum (G. Don) Hepper

• Pelecanus minor Gmelin 1789, the Great Frigatebird
  • Fregata minor