species , n
8. a. A class composed of individuals having some common qualities or characteristics, freq. as a subdivision of a larger class or genus.
10. Zool . and Bot . A group or class of animals or plants (usually constituting a subdivision of a genus) having certain common and permanent characteristics which clearly distinguish it from other groups. The exact definition of a species, and the criteria by which species are to be distinguished (esp. in relation to genera or varieties), have been the subject of much discussion.

-- Oxford English Dictionary, 2nd edn

More than a century after Darwin, there are still serious debates among biologists (and even more so among philosophers of biology) about how to define species . Shouldn't scientists define their terms? Yes, of course, but only up to a point. It turns out that there are different species concepts with different uses in biology -- what works for paleontologists is not much use to ecologists, for instance -- and no clean way of uniting them or putting them in an order of importance that would crown one of them (the most important one) as the concept of species. So I am inclined to interpret the persisting debates as more a matter of vestigial Aristotelian tidiness than a useful disciplinary trait.

Darwin's Dangerous Idea

, 1995
[on the search for 'necessary and sufficient conditions']