The Imperial Senate was a legislature that existed within the Terran Empire of the mirror universe. It served as a counterbalance to the power held by the Emperor though a great many rulers arose from the ranks of the Senate itself. Those that walked its corridors were given the opportunity to form the adequate allegiances needed to bring about a claim for the throne itself. (Decipher RPG module: Through a Glass, Darkly)
Christopher Pike was an Imperial Senator. (TOS comics: "Mirrored, Part 1", "Mirrored, Part 2")
In 2278, the Senate elected Spock to a five year term as the Commander in Chief of the Terran Empire. (TOS novel: Spectre)