'chief of the court, chief justice'[1]), abbreviated abd (אב״ד avad), was the second-highest-ranking member of the Sanhedrin during the Second Temple period and served as an assistant to the nasi.
[2] The av beit din was known as the "Master of the Court;" he was considered the most learned and important of these seventy members.
The Jerusalem Talmud tells the story of how Gamaliel II was deposed and Eleazar ben Azariah replaced him as Nasi.
[9] The holder of the title rosh beth din (Hebrew: ראש בית דין, lit.
Federation of Synagogues' Yisroel Yaakov Lichtenstein used this title when he published a major response in 2009, even though he was ABD.