Groups of populations (which are groups of individuals living together that are separated from other such groups) which can potentially interbreed or are actually interbreeding, that can successfully produce viable, fertile
offspring (without the help of
human technology).
Ernst Mayr, 1969. The species is the most fundamental unit of evolution and is the most specific taxonomic level.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.