The International Code of Zoological Nomenclature (ICZN) is a widely accepted convention in zoology that rules the formal scientific naming of organisms treated as animals. It is also informally known as the ICZN Code, for its formal author, the International Commission on Zoological Nomenclature (which shares the initialism "ICZN"). The rules principally regulate: How names are correctly established in the frame of binominal nomenclature How to determine whether a given name is available Which available name must be used in case of name conflicts (valid name) How scientific literature must cite names Zoological nomenclature is independent of other systems of nomenclature, for example botanical nomenclature. This implies that animals can have the same generic names as plants (e.g. there is a genus Abronia in both animals and plants).

The rules and recommendations have one fundamental aim: to provide the maximum universality and continuity in the naming of all animals, except where taxonomic judgment dictates otherwise. The code is meant to guide only the nomenclature of animals, while leaving zoologists freedom in classifying new taxa. In other words, while species concepts (and thus the definition of species) are arbitrary to some degree, the rules for names are not. The code applies only to names. A new animal 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 which available names are valid for any taxon in the family group, genus group, and species group. It has additional (but more limited) provisions on names in higher ranks. The code recognizes no case law. Any dispute is decided first by applying the code directly, and not by reference to precedent.

The code is also retroactive or retrospective, which means that previous editions of the code, or previous other rules and conventions have no force anymore today, and the nomenclatural acts published earlier must be evaluated only under the present edition of the code. In cases of disputes a case can be brought to the Commission who has the right to publish a final decision.

Principles In regulating the names of animals it holds by six central principles, which were first set out (as principles) in the third edition of the code (1985):

Principle of binominal nomenclature

This is the principle that the scientific name of a species, and not of a taxon at any other rank, is a combination of two names; the use of a trinomen for the name of a subspecies and of uninominal names for taxa above the species group is in accord with this principle.

This means that in the system of nomenclature for animals, the name of a species is composed of a combination of a generic name and a specific name; together they make a "binomen". No other rank can have a name composed of two names. Examples: Species Giraffa camelopardalis Subspecies have a name composed of three names, a "trinomen": generic name, specific name, subspecific name: Subspecies Giraffa camelopardalis rothschildi Taxa at a rank above species have a name composed of one name, a "uninominal name". Genus Giraffa, family Giraffidae In botanical nomenclature, the equivalent for "binominal nomenclature" is "binary nomenclature" (or sometimes "binomial nomenclature").

Principle of priority

This is the principle that the correct formal scientific name for an animal taxon, the valid name, correct to use, is the oldest available name that applies to it. It is the most important principle—the fundamental guiding precept that preserves zoological nomenclature stability. It was first formulated in 1842 by a committee appointed by the British Association to consider the rules of zoological nomenclature. Hugh Edwin Strickland wrote the committee's report.

Examples:

Nunneley 1837 established Limax maculatus (Gastropoda), Wiktor 2001 classified it as a junior synonym of Limax maximus Linnaeus, 1758 from S and W Europe. Limax maximus was established first, so if Wiktor's 2001 classification is accepted, Limax maximus takes precedence over Limax maculatus and must be used for the species.

Carl 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 a single species the principle of priority means that the species ought to be named F. catus, but in practice almost all biologists have used F. silvestris. In 2003, the Commission issued a ruling (Opinion 2027) 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. Taxonomists who consider the domesticated cat the same species as the wild cat should use F. silvestris; those who consider it a subspecies of the wild cat should use F. silvestris catus; those who consider it a separate species should use F. catus.

There are over 2 million junior synonyms recorded in zoology, primarily at the species level.

Principle of coordination

The principle of coordination is that within the family group, genus group and species group, a name established for a taxon at any rank in the group is simultaneously established with the same author and date for taxa based on the same name-bearing type at other ranks in the corresponding group. In other words, publishing a new zoological name automatically and simultaneously establishes all corresponding names in the relevant other ranks with the same type.

In the species-group, publishing a species name (the binomen) Giraffa camelopardalis Linnaeus, 1758 also establishes the subspecies name (the trinomen) Giraffa camelopardalis camelopardalis Linnaeus, 1758. The same applies to the name of a subspecies; this establishes the corresponding species name, should anyone choose to elevate it to a full species at a later date.

In the genus-group, similarly, publishing the name of a genus also establishes the corresponding name of a subgenus (or vice versa): genus Giraffa Linnaeus, 1758 and subgenus Giraffa (Giraffa) Linnaeus, 1758.

In the family-group, publication of the name of a family, subfamily, superfamily, tribe (or any other such rank) also establishes the names in all the other ranks in the family group (family Giraffidae, superfamily Giraffoidea, subfamily Giraffinae, tribe Giraffini).

Author citations for such names (for example a subgenus) are the same as for the name actually published (for example a genus). It is immaterial if there is an actual taxon to which the automatically established name applies; if ever such a taxon is recognised, there is a name available for it. Later publications of similar names based on the same type but with different endings (e.g., "-inae" for subfamily versus "-idae" for family) are homonyms.

Principle of the First Reviser

This is the principle that in cases of conflicts between simultaneously published divergent acts, the first subsequent author can decide which has precedence. It supplements the principle of priority, which states that the first published name takes precedence. The principle of the First Reviser deals with situations that cannot be resolved by priority. These items may be two or more different names for the same taxon, two or more names with the same spelling used for different taxa, two or more different spellings of a particular name, etc. In such cases, the first subsequent author who deals with the matter and chooses and publishes the decision in the required manner is the First Reviser, and their choice is to be followed.

Example:

Linnaeus 1758 established Strix scandiaca and Strix noctua (Aves), for which he gave different descriptions and referred to different types, but both taxa later turned out to refer to the same species, the snowy owl. The two names are subjective synonyms. Lönnberg 1931 acted as First Reviser, citing both names and selecting Strix scandiaca to have precedence.

Principle of homonymy

This is the principle that the name of each taxon must be unique. Consequently, a name that is a junior homonym of another name must not be used as a valid name.

It means that any one animal name, in one particular spelling, may be used only once (within its group). This is usually the first-published name; any later name with the same spelling (a homonym) is barred from being used. The principles of priority and first reviser apply here. For family-group names the termination (which is rank-bound) is not taken into account.

Genera are homonyms only if exactly the same — a one-letter difference is enough to distinguish them.

Examples: Argus Bohadsch, 1761 (Gastropoda) (was made available for homonymy by ICZN in Opinion 429, despite that Bohadsch 1761 was non-binominal - this had the effect that none of the various following names Argus can be used for a taxon) Argus Scopoli, 1763 (Lepidoptera: Lycaenidae: Polyommatinae) Argus Scopoli, 1777 (Lepidoptera: Nymphalidae: Satyrinae) Argus Poli, 1791 (Bivalvia) Argus Temminck, 1807 (Aves) Argus Lamarck, 1817 (Lepidoptera: Hesperiidae) Argus Walckenaer, 1836 (Araneae) Argus Gerhard, 1850 (Lepidoptera: Lycaenidae: Theclinae) The following are not homonyms of Argus: Argua Walker, 1863 (Lepidoptera), Argusa Kelham, 1888 (Aves), Argusina Hebard, 1927 (Dermaptera), †Arcus Hong, 1983 (Diptera), Argas Latreille, 1795 (Araneae), Argulus Müller, 1785 (Crustacea). The following names are not homonyms of each other: Isomya Cutler & Cutler, 1985 (Sipunculida), Isomyia Walker, 1859 (Diptera). Adelomya Mulsant & Verreaux, 1866 (Aves), Adelomyia Bonaparte, 1854 (Aves), †Adelomys Gervais, 1853 (Mammalia), †Adolomys Shevyreva, 1989 (Mammalia), Adulomya Kuroda, 1931 (Bivalvia).

Some spelling variants are explicitly defined by the Code as being homonyms. Otherwise the one-letter difference rule applies.

In species, primary homonyms are those with the same genus and same species in their original combination. The difference between a junior primary homonym and a subsequent use of a name is undefined, but it is commonly accepted that if the name referred to another species or form, gave a description, and if there is in addition no evidence the author knew that the name was previously used, it is considered as a junior homonym.

Example: Scopoli (1763) established Curculio fasciatus (Coleoptera) for a species from Slovenia. Strøm (1768) established Curculio fasciatus for another species from Norway. De Geer (1775) established Curculio fasciatus for a 3rd species from Sweden. Müller (1776) established Curculio fasciatus for a 4th species from Denmark. Fourcroy (1785) established Curculio fasciatus for a 5th species from France. Olivier (1790) established Curculio fasciatus for a 6th species from France. Marsham (1802) established Curculio fasciatus for a 7th species from Britain. All these names had descriptions that clarified that different species were meant, and that their authors did not know that the name had been established by a previous author.

Typically, junior primary homonyms are permanently invalid, but some are treated as valid if the junior and senior homonyms have been in separate genera after 1899 (Art. 57.2.1, Art. 23.9).

Examples: Olivier published the names Cerambyx elegans in 1790, and Cerambix [sic] maculatus in 1795, both of them junior primary homonyms. However, by 1835 both species were no longer in the genus Cerambyx, so both species names are presently treated as valid, as Mionochroma elegans and Stellognatha maculata, respectively.

Secondary homonyms occur when taxa with the same specific name but different original genera are later classified in the same genus (Art. 57.3, 59).