member of the House of Councillors
Refers to a member of the upper house of Japan's National Diet. Often used in political news and formal contexts.
He plans to run for a seat in the House of Councillors.
衆院議員 is a member of the House of Representatives (lower house), while 参院議員 is a member of the House of Councillors (upper house).
Compound of 参院 (House of Councillors) and 議員 (member of an assembly).