In biology, a homonym is a name for a taxon that is identical in spelling to another such name, that belongs to a different taxon.

The rule in the International Code of Zoological Nomenclature is that the first such name to be published is the senior homonym and is to be used (it is "valid"); any others are junior homonyms and must be replaced with new names. It is, however, possible that if a senior homonym is archaic, and not in "prevailing usage," it may be declared a nomen oblitum and rendered unavailable, while the junior homonym is preserved as a nomen protectum.

For example:

• Cuvier proposed the genus Echidna in 1797 for the spiny anteater.
• However, Forster had already published the name Echidna in 1777 for a genus of moray eels.
• Forster's use thus has priority, with Cuvier's being a junior homonym.
• Illiger published the replacement name Tachyglossus in 1811.

Similarly, the International Code of Nomenclature for algae, fungi, and plants (ICN) specifies that the first published of two or more homonyms is to be used: a later homonym is "illegitimate" and is not to be used unless conserved (or sanctioned, in the case of fungi).

Example: the later homonym Myroxylon L.f. (1782), in the family Leguminosae, is conserved against the earlier homonym Myroxylon J.R.Forst. & G.Forst. (1775) (now called Xylosma, in the family Salicaceae).