A generic term for a family of 209 chlorinated isomers of biphenyl. The biphenyl molecule is composed of two six-sided carbon rings connected at one carbon site on each ring. Ten sites remain for chlorine atoms to join the biphenyl molecule. While the rules of nomenclature would indicate that the term polychlorinated biphenyl can refer only to molecules containing more than one chlorine atom, the term has been used to refer to the biphenyl molecule with one to ten chlorine substitutions.