Av Beit Din

Av Beit Din, or Av Beis Din, or Av Beth Din (אב בית דין) Hebrew, literally "Father of the House of Justice". The Av Beit/Beis Din was the second-highest ranking member of the Sanhedrin during the Second Commonwealth period. He presided over the Sanhedrin in the absence of the Nasi, and was the chief of the Sanhedrin when it sat as a criminal court. He sat with seven other judges while hearing a case.

It is presently being used as an honorific title for the presiding rabbi of a beit/beis din (rabbinical court), who is typically the salaried rabbi of the local Jewish community and not necessarily a renowned posek ("decisor" of Jewish law.)