A one-party state is a form of government where the country is ruled by a single political party, meaning only one political party exists and the forming of other political parties is forbidden.
Some countries have many political parties that exist, but only one that can by law be in control, which is called a one-party dominant state. In this case opposition parties against the dominant ruling party are allowed, but have no real chance of gaining power. For example, in China all power is vested in the Communist Party of China. Other parties are allowed to exist only if they accept the leading role of the Communist Party.
The Soviet Union from 1922-1991, Nazi Germany from 1933-1945, Italy under Benito Mussolini from 1922-1943, and various Eastern Bloc states are some the best known examples of one-party states in history. Some one-party states are considered dictatorships and called a police state or a military dictatorship, if a secret police force or the military is used to keep a dictator in power through force.
As of April 2015, there are seven states that are ruled by a single party: