For the mathematical term, see topology
In biology, a genus is a group of living beings. The term was introduced with Linnaean taxonomy but has been retained by newer systems of classification and is still in universal currency. In the original Linnaean system, one or more species make up one genus, and one or more genera make up one family. Biologists have since introduced a level between genus and family, the subfamily. This addition was necessary because we know many times more species now than we used to know when the system was first invented, so families became extremely large and unwieldy.
The scientific name of every species consists of two words, the generic name and the specific name. The generic name is simply the name of the genus the species belongs to and is supposed to be unique across all of biology; no two genera may have the same name. Two or more species may share the same specific name, but no two such species may belong to the same genus. The specific name, in other words, uniquely identifies a species within a genus; in combination with the generic names it uniquely identifies a species.
- Certhia is the scientific name of the true treecreepers, a group of about a dozen species of small passerine birds living in forests on the Northern Hemisphere. Certhia familiaris is the scientific name of the common treecreeper, one particular species of true treecreepers. Certhia americana is the scientific name of the brown treecreeper, another species of true treecreepers.
- Acrocephalus is the scientific name of a genus of warblers sometimes called the marsh-warblers or reed-warblers. Acrocephalus familiaris is the scientific name of one particular species in this genus, the species of the millerbirds. Note that there are two species sharing one specific name, familiaris. This is permissible because their complete scientific names are still different. In fact, there are literally dozens of species called familiaris.
- Acrocephalus is, however, also the scientific name of a genus of mint plants. This is problematic as every genus name is supposed to point to one genus and one genus only.
The generic name and the specific name may actually be the same. This often happens in the case of type species.