The term species is used to describe a group of closely related, physically similar organisms that can interbreed.

In taxonomical classification of plants and animals, species is the most specific category, following in order kingdom, phylum (animals) or division (plants), class, order, family, and genus. Each identified species name is assigned a two part name, part of the binomial system popularized by Carl Linnaeus. The first part of the name is the genus of the species. The second part of the name is the specific epithet. The two parts together give a unique name for every type of living organism. The name is generally given in a Latinized form and there are rigid sets of rules to be followed in the description of a new species and the granting of a name. These rules are given in great detail in, for plants, The International Code of Botanical Nomenclature and for animals in The Zoological Code. Other descriptive rules exist for bacteria and other microorganisms.

The taxonomic species is the smallest unit of the classification system normally used. The common names associated with many organisms generally denote a particular species. Many individual groups show an amount of regional variation, which sets them apart form other members of the same species. These different groups are still capable of breeding with each other but they are given the rank of subspecies. The dividing lines between species or between species and subspecies are sometimes unclear.

In order to distinguish species, biologists search for recognized diagnostic characteristics. However, species differ from one another not only by conspicuous features, but also by habitat and genetic makeup. Members of the same species share a common gene pool, as well as biochemical, morphological, and behavioral characteristics.

A species population has a range, the habitat within which it lives and a dimension in time. Back in time, the species merges with other species, and in the future, it may branch into several species. If it's future is questionnable, the species is considered endangered. Extinction is a result of an imbalance between a species and its range.