A one-party state, single-party state, one-party system or single-party system is a governance structure in which only a single political party controls the ruling system. In a one-party state, all opposition parties are either outlawed or have limited and controlled participation in elections. The expression "de facto one-party" is sometimes used to describe a dominant-party system that, unlike a one-party state, allows multiparty elections, but the existing practices or balance of political power effectively prevent the opposition from winning power.