Use the adjective unicameral to describe a government with only one legislative house or chamber. The Parliament of Finland, for example, is unicameral.
Some governments are split into two houses — these are called bicameral legislatures. When there is only one house, usually because the government is small or the country is homogeneous, it's called unicameral. Having a unicameral government is simpler and more straightforward, while a bicameral legislature can represent more diverse groups. The word unicameral has two Latin roots, uni, which means "one," and camera, "chamber."