Av Beit Din, Av Beis Din, or Abh Beyth Diyn (Hebrew: אב בית דין, "Chief of the Court"). 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.
In modern times it is used as an honorific title for the presiding rabbi of a beth din (rabbinical court), who is typically the salaried rabbi of the local Jewish community and usually a posek ("decisor" of Jewish law.) It is also abbreviated as AB"D when it is appended after the name of the Chief Rabbi of a national Jewish community.