What is "biphenylene"

n. (context organic compound English) A tricyclic aromatic hydrocarbon formed from two benzene rings joined by two adjacent single bonds; any derivative of this compound

Biphenylene is an alternant, polycyclic hydrocarbon composed of two benzene rings joined together by a pair of mutual attachments (as opposed to a normal ring fusion), thus forming a 6-4-6 arene system. The resulting planar structure was one of the first pi-electronic hydrocarbon systems discovered to show evidence of antiaromaticity.

The biphenylene structure can also be understood as a dimer of the reactive intermediate benzyne, which in fact serves as a major synthetic route, e.g., from benzenediazonium-2-carboxylate.