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.