[¦päl·ə′nü·klē·ər ′hī·drə‚kär·bən] (organic chemistry) Hydrocarbon molecule with two or more closed rings; examples are naphthalene, C10H8, with two benzene rings side by side, or diphenyl, (C6H5)2, with two bond-connected benzene rings. Also known as polycyclic hydrocarbon.