(1) The lowest taxonomic rank, and the most basic unit or category of biological classification (note: in biological classification, taxonomic categories are, from "largest" or most inclusive to "smallest" or most specific - kingdom, phylum, class, order, family [and sometimes subfamily], genus, species).
(2) 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.
In binomial nomenclature, the species is used as the second word of a scientific name. The species name is typically italicized. Example: Cervus canadensis is the scientific name for elk, with Cervus referring to the genus of elk, and canadensis referring to the species name.
Definition from URL: http://www.biology-online.org/dictionary/Species