A senator was a member of a legislative or deliberative body known as a senate.
The Romulan Star Empire was one of the best known groups who utilized this style of legislative chamber, often, though not exclusively, seen in representative democracies. Romulan senators were elected to the Romulan Senate. Romulan senators represented districts. (TNG: "Unification I")
At least three Senators represented the Romulan Star Empire on Deep Space 9 during the Dominion War - Letant, Kimara Cretak, and Vreenak. (DS9: "Tears of the Prophets", "Image in the Sand", "Shadows and Symbols", "Inter Arma Enim Silent Leges", "In the Pale Moonlight")