noun, singular or plural: species
(1) (taxonomy) The lowest taxonomic rank, and the most basic unit or category of biological classification.
(2) (taxonomy) An individual belonging to a group of organisms (or the entire group itself) having common characteristics and (usually) are capable of mating with one another to produce fertile offspring. Failing that (for example the Liger) It has to be ecologically and recognisably the same.
In the hierarchy of biological classification, it is the lowest taxonomic rank and is considered as the most basic unit of classification. Genus is the taxonomic rank above the species and may contain one of more species. In order to be considered into a species rank, the group of organisms wherein two of its members are capable of reproducing fertile offspring (especially through sexual reproduction). There are certain groups though that can still be further subdivided into subgroups (i.e. subspecies, such as varieties, formae, etc.).
A species is given a two-part name: the generic name and the specific name (or specific epithet). For example, Allium cepa (commonly known as onion). The Allium is the generic name whereas the cepa is the specific epithet.
Word origin: Middle English, logical classification, from Latin speciēs, a seeing, kind, form”.