Sanhedrin are ancient Jewish legal and religious institutions in Jerusalem. Supposedly started by Moses. It is the court that sentenced Jesus. There were two Sanhedrins one a legal,political and civil court and the Great Sanhedrin a religious court. As far as I can tell the distinction between the two courts and even the existence of two seperate courts is shady and it seems there was alot of crossover and influence from the The Great Sanhedrin into the legal aspects of Jewish life. The political Sanhedrin perished after the destruction of the Temple in A.D. 70, but the religious Sanhedrin continued as the rabbinic patriarchate until about A.D. 425. The legal Sanhedrins was far ahead of its times in terms of complexity. if you want to know more read The Trial of Jesus.