The Collaborative International Dictionary of English v.0.48:

Sanhedrin \San"he*drin\, Sanhedrim \San"he*drim\, n. [Heb.
   sanhedr[imac]n, fr. Gr. ?; ? with + ? a seat, fr. ? to sit.
   See Sit.] (Jewish Antiq.)
   the great council of the Jews, which consisted of seventy
   members, to whom the high priest was added. It had
   jurisdiction of religious matters.
   [1913 Webster]

Easton's 1897 Bible Dictionary:

Sanhedrim
   more correctly Sanhedrin (Gr. synedrion), meaning "a sitting
   together," or a "council." This word (rendered "council," A.V.)
   is frequently used in the New Testament (Matt. 5:22; 26:59; Mark
   15:1, etc.) to denote the supreme judicial and administrative
   council of the Jews, which, it is said, was first instituted by
   Moses, and was composed of seventy men (Num. 11:16, 17). But
   that seems to have been only a temporary arrangement which Moses
   made. This council is with greater probability supposed to have
   originated among the Jews when they were under the domination of
   the Syrian kings in the time of the Maccabees. The name is first
   employed by the Jewish historian Josephus. This "council" is
   referred to simply as the "chief priests and elders of the
   people" (Matt. 26:3, 47, 57, 59; 27:1, 3, 12, 20, etc.), before
   whom Christ was tried on the charge of claiming to be the
   Messiah. Peter and John were also brought before it for
   promulgating heresy (Acts. 4:1-23; 5:17-41); as was also Stephen
   on a charge of blasphemy (6:12-15), and Paul for violating a
   temple by-law (22:30; 23:1-10).

     The Sanhedrin is said to have consisted of seventy-one
   members, the high priest being president. They were of three
   classes (1) the chief priests, or heads of the twenty-four
   priestly courses (1 Chr. 24), (2) the scribes, and (3) the
   elders. As the highest court of judicature, "in all causes and
   over all persons, ecclesiastical and civil, supreme," its
   decrees were binding, not only on the Jews in Palestine, but on
   all Jews wherever scattered abroad. Its jurisdiction was greatly
   curtailed by Herod, and afterwards by the Romans. Its usual
   place of meeting was within the precincts of the temple, in the
   hall "Gazith," but it sometimes met also in the house of the
   high priest (Matt. 26:3), who was assisted by two
   vice-presidents.