Acts 5:21 - "Now the high priest came and those who were with him and called together the council and all the senate of Israel, and sent to the prison to have them brought."
Psalm 105:22 - "To bind his princes at his pleasure; and teach his senators wisdom" (KJV).
The quotation from the Psalms is the only place in the Old Testament where we find the use of the term "senator;" generally the Hebrew uses the word "elder." It may have been this use of the term which made Luke think that there was a separate branch of the Great Sanhedrin which was composed of "senators." However, although some have thought that the Sanhedrin was in fact composed of three bodies - senators, scribes, and priests - it was, in fact, an entity, always considered as a whole, not separate factions.
However, again the apocryphal writings do use it rather more frequently; according to II Macc 6:1, Antiochus Epiphanes sent an "Athenian senator" to the Jews to force their acceptance of the worship of Olympian Zeus. The earliest historical writings to use the term in referring to members of the Great Sanhedrin come from Josephus, that fount of ancient knowledge - in Antiq. XII. iii. 3, again it is Antiochus the Great who uses the term.
We can, then, by gathering together all these divergent uses, conclude that the term was at least one of respect, indicating a person of rank within the legal and judicial system of the Jews. We can easily bridge the time gap and consider the "senator" today as a member, perhaps, of a supreme court or other law-making and law-enforcing body. Just as the term implies a person of wisdom, experience, and sound judgment, so we think of the officials whom we elect to be the arbiters and maintainers of our way of life. We, of course, still use the term in connection with the members of our state and federal governmental systems. And, hopefully, the men that we elect to fill these positions are also men of wisdom and judgment.