Sejm, lower house of the national legislature of Poland. The term Sejm initially referred to the Polish legislature as a whole, which first met for all of Poland in 1493 and historically thereafter usually comprised two houses. In 1946 the Senate, or upper house, of the body was eliminated. It was not reinstated until 1989, when the name Sejm came to refer only to the lower house, which plays a more dominant role than the Senate.