This essay argues that aristotle's doctrine of nominal definition is his semantic theory for natural-Kind terms. It offers a new interpretation of that doctrine. On this interpretation nominal definitions are initial working theoretical accounts of natural kinds which serve as starting points for scientific inquiry. As such, Nominal definitions have existential import. They make an implicit reference to the most familiar actual instances of the kinds they define and they define the essences of those kinds by reference to those instances. (...) Nominal definitions are, Thus, Accounts of real essences as much as real definitions. They are initial incomplete accounts of such essences and it is as such that they explicate the meanings of (natural-Kind) terms. (shrink)