polychlorinated biphenyl

any of a class of organohalogen compounds prepared by the reaction of chlorine with biphenyl. A typical mixture of PCBs may contain over 100 compounds and is a colourless, viscous liquid. The mixture is relatively insoluble in water, is stable at high temperatures, and is a good dielectric (electrical insulator). Because of these qualities, PCBs are particularly useful as lubricants, heat-transfer fluids, and fire-resistant insulating fluids in transformers and capacitors.