This article discusses the many usages and meanings of the term species in biology, as currently applied (1) to named taxa such as Homo sapiens and Panthera tigris, (2) to a rank (usually but not necessarily the lowest and/or the most fundamental one) of the biological classification, and (3) as a variegated set of notions, the most important among them being the morphospecies, the biospecies, the ecospecies, the evolutionary species, the agamospecies and the taxonomic species. The debatable relationship between the Aristotelian “definition by species and genus” and the historical roots of the usage of genus and species as basic ranks in biological classification are outlined, as are the historical roots of the scientific nomenclature applied to biological species.