From BiblePortal Wikipedia

Webster's Dictionary [1]

(1): ( n.) The upper and less numerous branch of a legislature in various countries, as in France, in the United States, in most of the separate States of the United States, and in some Swiss cantons.

(2): ( n.) In general, a legislative body; a state council; the legislative department of government.

(3): ( n.) The governing body of the Universities of Cambridge and London.

(4): ( n.) In some American colleges, a council of elected students, presided over by the president of the college, to which are referred cases of discipline and matters of general concern affecting the students.

(5): ( n.) A body of elders appointed or elected from among the nobles of the nation, and having supreme legislative authority.

(6): ( n.) An assembly or council having the highest deliberative and legislative functions.

Vine's Expository Dictionary of NT Words [2]

1: Γερουσία (Strong'S #1087 — Noun Feminine — gerousia — gher-oo-see'-ah )

"a council of elders" (from geron, "an old man," a term which early assumed a political sense among the Greeks, the notion of age being merged in that of dignity), is used in  Acts 5:21 , apparently epexegetically of the preceding word sunedrion, "council," the Sanhedrin.

King James Dictionary [3]

SEN'ATE, n. L. senatus, from senex, old.

1.An a assembly or council of senators a body of the principal inhabitants of the city or state,

with a share in the government. The senate of ancient Rome was one of the most illustrious bodies of men that ever bore this name. Some of the Swiss cantons have a senate, either legislative or executive.

2. In the United States, senate denotes the higher branch or house of legislature. Such is the senate of the United States, or upper house of the congress and in most of the states, the higher and least numerous branch of the legislature, is called the senate. In the U. States, the senate is an elective body. 3. In a looser sense, Any legislative or deliberative boky of men as the eloquence of the senate.

Hastings' Dictionary of the Bible [4]

Senate is the tr. [Note: translate or translation.] of Gr. gerousia in   Acts 5:21 , where ‘all the senate of the children of Israel’ is intended to explain the preceding ‘ council ’ ( synedrion ). See Sanhedrin. It is the Jewish ‘senate’ that is meant likewise in   Malachi 1:10  Malachi 1:10; 2Ma 4:44 . The Roman senate is alluded to in 1Ma 8:17 ff.

American Tract Society Bible Dictionary [5]

 Acts 5:21 . See Sanhedrin .

Holman Bible Dictionary [6]

 Acts 5:21Sanhedrin

Easton's Bible Dictionary [7]

 Acts 5:21

Hastings' Dictionary of the New Testament [8]

See Sanhedrin.

Cyclopedia of Biblical, Theological and Ecclesiastical Literature [9]

( Γερουσία , Eldership, used by classical writers for a deliberative or legislative body, and by the Sept. for the collective mass of the Jewish elders, and later for the Sanhedrim) is used once in the New Test. ( Acts 5:21) for some portion of the Sanhedrim, apparently the elders, who constituted its main element. (See Elder); (See Sanhedrim).

The Nuttall Encyclopedia [10]

E . "an assembly of elders"), a name first bestowed by the Romans on their supreme legislative and administrative assembly; its formation is traditionally ascribed to Romulus; its powers, at their greatest during the Republic, gradually diminished under the Emperors; in modern times is used to designate the "Upper House" in the legislature of various countries, e. g . France and the United States of America; is also the title of the governing body in many universities.