1. A group of organisms that resemble each other more than they resemble members of other groups and cannot be subdivided into two or more species. The precise definition of what constitutes a species differs depending on which species concept is applied. According to the biological species concept, a species comprises a group of individuals that can usually breed among themselves and produce fertile offspring. However, many other species concepts have been proposed, including the phylogenetic species concept and various typological species concepts. Typically, a species consists of numerous local populations distributed over a geographical range. Within a species, groups of individuals become reproductively isolated because of geographical or behavioural factors (see isolating mechanism), and over time may evolve different characteristics and form a new and distinct species.

2. A rank, or category, used in the classification of organisms. Similar species are grouped into a genus, and a single species may be subdivided into subspecies or races. See also binomial nomenclature.