Av Beit Din

The av beit din (Hebrew: אָב בֵּית דִּין ʾabh bêth dîn, "chief of the court" or "chief justice"), also spelled av beis din or abh beth din and abbreviated ABD (אב״ד‎), was the second-highest-ranking member of the Sanhedrin during the Second Temple period, and served as an assistant to the Nasi (Prince). The Av Beit Din was known as the "Master of the Court;" he was considered the most learned and important of these seventy members.

Menahem the Essene served as Av Beth Din in the 1st century BCE, before abdicating to "serve the King" in 20 BCE. The House of Shammai attained complete ascendency over the Sanhedrin from 9CE until Gamaliel became Nasi in 30CE. Apparently the post of Av Beit Din was eventually filled, since the Babylonian Talmud states that Joshua ben Hananiah was Av Beit Din and Nathan the Babylonian was Av Beit Din. The Jerusalem Talmud tells the story of how Gamaliel II was deposed and Eleazar ben Azariah replaced him as Nasi. After Gamaliel was reinstated, Eleazar ben Azariah was made Av Beit Din. The parallel story in the Babylonian Talmud has Eleazar ben Azariah remaining as a co-Nasi with Gamaliel.