A senator is an official who sits in a senate.
The Romulan Star Empire was governed by the Romulan Senate. (TNG: "Unification I") Romulan senatorial seats were hereditary except in cases of dishonor or where a senator died with no clear heir. (TOS novel: The Romulan Way)
The death of the Romulan senator Merken tr'Vreenak was a significant moment in the Dominion War, leading to the Romulan Star Empire to enter the war and joining the Federation Alliance. (DS9: "In the Pale Moonlight", The Dominion War Sourcebook: The Fires of Armageddon)