Sejm was the name of the Polish parliament before the 20th century, which consisted of the lower house (Izba Poselska) and the upper house (Senat) and the King. It was usually referred as 3 states parliament.
The history of the Sejm dates back to 1493. The indirect elections were repeated every two years.
Since 1918, the name Sejm has referred only to the lower house of the Parliament; the upper house is still the Senate - Senat. Since the fall of Communism, the Sejm comprises 460 members elected in proportional elections every 4 years.
See also: Politics of Poland