A communist state is a state ruled by a single-party which declares its allegiance to Marxism-Leninism. The term communist state originated from the fact that most of the states in question were or are ruled by parties that called themselves "Communist Party of [country]". Thus, they became known as Communist Party-ruled states, or simply communist states. However most of these states called themselves socialist, since in Marxist political theory, socialism is the intermediate stage in reaching communism, which is a condition with no state, so that communist state is considered an oxymoron.
Communist governments, or governments that were - or claimed to be - moving in a communist direction, tend to have arose in times of political turmoil. Often the establishment of a communist government follows a communist revolution, where dissatisfied members of the working class rise against the government.
Defunct Communist states and their ruling parties (where applicable):