Species is a natural population or group of natural populations of individuals which are genetically distinct and reproductively isolated with similar essential morphological traits.

Species is also a genetically closed system because its members do not interbreed with members of other species. Species is lowest or basic taxonomic category, e.g., mango (Mangifera indica), potato (Solanum tuberosum), lion (Panthera leo).

Here indica, tuberosum, leo are species of genera Mangifera, Solanum and Panthera respectively. All other taxonomic categories are defined and described in relation to species.

For example, a genus is a group of species and a subspecies or a variety is a part of species. New species originate from already existing species. Species is considered basic unit of taxonomy since in the greater majority of cases we do not have intraspecific names.