Av Beit Din

Av Beit Din, Av Beis Din, or Abh Beyth Diyn, (abbreviated: AB"D, (Hebrew: אב בית דין‎, "Chief of the Court"). was the second-highest ranking member of the Sanhedrin during the Second Commonwealth period. The president, who bore the title Nasi, was in a way the supervisor, but not a member of the court, which consisted of seventy members. The most learned and important of these seventy members was called Av Beit Din, a title similar to that of vice-president.

The Av Beit Din 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.