γερουσία, -ας, ἡ (< γέρων), [in LXX (Hex. only in OT) for זָקֵן ;] a council of elders, senate; in NT, of the Sanhedrin (Cl. Rev., i, 43f.; DB, ext., 99): Act.5:21.† (AS)
Thayer:
1) a senate, council of elders 1a) to denote the chief council of nations or cities 1b) the Sanhedrin
γερουσία
gerousia
gher-oo-see'-ah
From G1088; the eldership, that is, (collectively) the Jewish Sanhedrim