The Collaborative International Dictionary of English v.0.48:

Naphthene \Naph"thene\, n. (Chem.)
   A peculiar hydrocarbon fraction occuring as an ingredient of
   some crude petroleums, mostly mixtures of derivatives of the
   five- and six-membered saturated cyclic alkanes, cyclopentane
   and cyclohexane, having the general formula CnH2n.
   [1913 Webster +PJC]