A genus is

1. a low-level taxonomic rank used in the classification of living and fossil organisms. Other well-known taxonomic ranks are domain, kingdom, phylum, class, order, family, and species, with genus fitting between family and species. Or
2. a taxonomic unit (a taxon), in that rank. In this case the plural is genera.

The term comes from Latin genus "descent, family, type, gender" (plurals: genera), cognate with - genos, "race, stock, kin".

Like for the other well-known taxonomic ranks, mentioned above, there is an immediately lower rank, indicated by the prefix sub-, in this case subgenus, plural subgenera. The most important taxonomic unit below the genus is the species, which is the basic rank.

The composition of each genus is determined by a taxonomist, but often there is no exact agreement, with different taxonomists each taking a different position. There are no hard rules that a taxonomist needs to follow in describing a genus, but see below for some rules of thumb.

Generic name

The scientific name of a genus may be called the generic name: it is always capitalized. It plays a pivotal role in binomial nomenclature, the system of biological nomenclature.

Binomial nomenclature

The rules for scientific names are laid down in the Nomenclature Codes; depending on the kind of organism and the Kingdom it belongs to, a different Code may apply, with different rules, laid down in a different terminology. The advantages of scientific over common names are that they are accepted by speakers of all languages, and that each species has only one name. This reduces the confusion that may arise from the use of a common name to designate different things in different places (example elk), or from the existence of several common names for a single species.

It is possible for a genus to be assigned to a kingdom governed by one particular Nomenclature Code by one taxonomist, while other taxonomists assign it to a kingdom governed by a different Code, but this is the exception, not the rule.

Pivotal in binomial nomenclature

The generic name often is a component of the names of taxa of lower rank. For example, Canis lupus is the scientific name of the Gray wolf, a species, with Canis the generic name for the dog and its close relatives, and with lupus particular (specific) for the wolf (lupus is written in lower case). Similarly, Canis lupus familiaris is the scientific name for the domestic dog.

Taxonomic units in higher ranks often have a name that is based on a generic name, such as the family name Canidae, which is based on Canis. However, not all names in higher ranks are necessarily based on the name of a genus: for example, Carnivora is the name for the order to which the dog belongs.

The problem of identical names used for different genera