In biological nomenclature, a type species is the species to which the name of a genus is permanently linked; it is the species that contains the biological type specimen(s) of the taxon. A type species is both a concept and a practical system which is used in the classification and nomenclature (naming) of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. This is an important concept whenever a taxon containing multiple species must be divided into more than one genus; the type species automatically assigns the name of the original taxon to one of the resulting new taxa, thus reducing the potential for confusion.
Under both the zoological and botanical nomenclature codes, every named genus or subdivision of a genus, whether or not currently recognized as valid or correct, should have a type species. In practice however there is a backlog of untypified names.
A similar concept is used for suprageneric groups, the concept of a type genus.
In zoology 
The term "type species" is regulated in zoological nomenclature by article 42.3 of the International Code of Zoological Nomenclature, which defines a type species as the name-bearing type of the name of a genus or subgenus (a "genus-group name") is the "type species". In the Glossary, type species is defined as
- "The nominal species that is the name-bearing type of a nominal genus or subgenus".
The type species permanently attaches a genus to its formal name (its generic name) by providing just one species within that genus to which the genus is permanently linked (i.e. the genus must include that species if it is to bear the name). The species name in turn is fixed, in theory, to a type specimen.
For example, the type species for the land snail genus Monacha is Monacha cartusiana. That genus is currently placed within the family Hygromiidae. The type genus for that family is the genus Hygromia.
In botany 
In botanical nomenclature, the type for any given taxonomic name - if it has a type - is technically a specimen (or illustration). In the case of the name of a genus (or of a subdivision of a genus), its type will usually be the type for a species included within it and can be indicated by the name of this species alone. The term "type species", although of no formal standing under the Code, is sometimes borrowed from zoological nomenclature.