International Code of Zoological Nomenclature

The International Code of Zoological Nomenclature (ICZN) is a set of rules in zoology that have one fundamental aim: to provide the maximum universality and continuity in the naming of all animals according to taxonomic judgment. The Code is meant to guide only the nomenclature of animals, while leaving the zoologists some degree of freedom in classifying new species and higher-level taxa. In other words, whether a species itself is or is not a real entity is a subjective decision, but what name should be applied to it is not; the Code applies only to the latter, not to the former. A new taxon name published without adherence to the Code may be deemed simply "unavailable" if it fails to meet certain criteria, or fall entirely out of the province of science (e.g., the "scientific name" for the Loch Ness Monster).

The rules in the Code determine what names are potentially valid for any taxon including the ranks of subspecies and superfamily. Its provisions can be waived or modified in their application to a particular case when strict adherence would cause confusion. Such exceptions are not made by an individual scientist, no matter how well-respected within his or her field, but only by the International Commission on Zoological Nomenclature, acting on behalf of all zoologists. The Commission takes such action in response to proposals submitted to it. Note that, formally, the acronym "ICZN" refers to the Commission, and not the Code. Misapplications of the acronym are pervasive, however, and even taxonomists will use the acronym occasionally when referring to the Code.

The Code recognizes no case law. Any dispute is decided by applying the Code directly, and never by reference to precedent.

Rules and examples of their application
The first published name of an organism or group takes priority; later names for that organism or group are junior synonyms and are not considered valid.
 * 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 synonym.
 * Johann Jakob Kaup published the name Leptocephalus brevirostris in 1856 for a species of eel. However, it was realized in 1893 that the organism described by Kaup was in fact the juvenile form of the European eel (see eel life history for the full story). The European eel was named Muraena anguilla by Carolus Linnaeus in 1758 and moved to the genus Anguilla by Franz Paula von Schrank in 1798. So Anguilla anguilla is now the valid name for the species, and Leptocephalus brevirostris is considered a junior synonym.

The first published use of a name takes priority; later uses of a name spelled the same but used to refer to different organisms are junior homonyms and must be given replacement names.
 * Georges Cuvier proposed the genus Echidna in 1797 for the spiny anteater. However, Johann Reinhold Forster had 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; Johann Karl Wilhelm Illiger published the name Tachyglossus in 1811 and this is considered to be the valid replacement name, or nomen novum.

The first published description of a species fixes the species epithet; if the species is later moved to another genus, it retains the first-published epithet unless that would create a homonym.
 * The Common Chimpanzee was named Simia troglodytes by Johann Friedrich Blumenbach in 1799; Lorenz Oken moved it to the new genus Pan in 1816, so the valid name is now Pan troglodytes.
 * Two species of Madagascar snake were given the species epithet madagascariensis by André Marie Constant Duméril and Gabriel Bibron in 1844 — Pelophilus madagascariensis and Xiphosoma madagascariensis. George Albert Boulenger moved the former to the genus Boa in 1893, giving it the name Boa madagascariensis. This meant that when Arnold G. Kluge of the Museum of Zoology at the University of Michigan moved Xiphosoma madagascariensis to the genus Boa in 1991, the name Boa madagascariensis was invalid — a junior secondary homonym. So Kluge gave the species the replacement name Boa manditra. This example also demonstrates a case where rules of gender agreement come into play (a tradition which has become more controversial in recent years). That is, an adjectival species epithet must - with very few exceptions - agree in gender with the name of the genus in which it is placed. If a species is moved, therefore, often it must have its spelling changed as a result. The genus name Xiphosoma is neuter in gender, and therefore the original spelling of the species should have been madagascariense, which is the neuter form - the spelling change to madagascariensis would occur only after being placed in Boa. Epithets that are nouns, or arbitrary combinations of letters, are not changed, but this is not always obvious from the appearance of a name, as in manditra, which is a noun, and would not change if, for example, it were moved to the genus Pelophilus (it would become Pelophilus manditra and not Pelophilus manditrus). Changes in placement, or confusion over proper Latin grammar, lead to many incorrectly-formed names appearing in print, and automated searches failing to find all the variant spellings of a given name (e.g., the epithets atra and ater may refer to the same species). Accordingly, many laymen and some scientists object to the continued adherence to this long-standing practice.

In the interests of stability of nomenclature, the rule of priority can be reversed if a junior name has been used very widely and for a long period of time.
 * Carolus Linnaeus named the Domestic Cat Felis catus in 1758; Johann Christian Daniel von Schreber named the Wildcat Felis silvestris in 1775. For taxonomists who consider these two kinds of cat to be a single species rule of priority means that the species ought to be named F. catus but in practice almost all biologists have used F. silvestris for the wild cat. In 2003, the Commission issued a ruling that "conserved the usage of 17 specific names based on wild species, which are pre-dated by or contemporary with those based on domestic forms", confirming F. silvestris for the wild cat, as a nomen protectum. Taxonomists who consider the domesticated cat a subspecies of the wild cat should use F. silvestris catus; the name F. catus remains available for the domestic cat where it is considered to be a separate species. See Opinion 2027 for 16 other species names conserved for the same reason.