Terphenyls are a group of closely related aromatic hydrocarbons. Also known as diphenylbenzenes or triphenyls, they consist of a central benzene ring substituted with two phenyl groups. The three isomers are ortho-terphenyl, meta-terphenyl, and para-terphenyl. Commercial grade terphenyl is generally a mixture of the three isomers. This mixture is used in the production of polychlorinated terphenyls, which were formerly used as heat storage and transfer agents.