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From 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]