Thayer's Greek Lexicon

 1. any assembly (esp. of magistrates, judges, ambassadors),
whether convened to deliberate or pass judgment
2. any session or assembly or people deliberating or adjudicating
  1) the Sanhedrin, the great council at Jerusalem, consisting of
the seventy one members, viz. scribes, elders, prominent
members of the high priestly families and the high priest, the
president of the assembly. The most important causes were
brought before this tribunal, inasmuch as the Roman rulers of
Judaea had left to it the power of trying such cases, and also
of pronouncing sentence of death, with the limitation that a
capital sentence pronounced by the Sanhedrin was not valid
unless it was confirmed by the Roman procurator.
  2) a smaller tribunal or council which every Jewish town had
for the decision of less important cases.