Type species is a term which is used in the taxonomy of organisms. The phrase and concept is used much more often and much more formally in zoology than it is in botany.
In zoology, the "type species" for any one particular genus fixes or 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. Thus that genus is clearly defined, and can be understood in more detail when necessary by examining the type material for that individual species.
The reason this is a helpful concept is that within one genus which may contain many species, it can happen that one or more species may at some point be moved to another genus. Anchoring the genus to one species rather than a group of species makes for less confusion. There is a similar system for tying a family to one genus, see Type genus.
For example, the type species for the land snail genus Monacha is Monacha cartusiana. That genus is currently placed in the family Hygromiidae. The type genus for that family is the genus Hygromia.
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The term type species is regulated in zoological nomenclature by article 42.3 of the International Code of Zoological Nomenclature, which defines 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 species name in turn is fixed to a type specimen. Ideally, every named genus or subgenus should have a type species, but in practice there is a backlog of untypified names.
In botany "type species" is an informal phrase only. In botanical nomenclature, the type of a name, of a genus or otherwise, is a specimen (or illustration) (ICBN, articles 10.1, 8.1 and 10.4). In the case of the name of a genus (or of a subdivision of a genus) this type will usually be the type of an included species, and can be indicated by the name of this species (Art 10.1). This species is sometimes informally called the "type species", but this phrase has no formal standing in botany.
In modern nomenclature, the type of a genus (often referred to as the generitype) is systematically that of a species, and thus only a species name is cited. As a result, despite the provisions of the code, it is not unusual to designate a genus' type without regard as to whether that species itself has a type (be in an holotype, lectotype or neotype).