A dissident, broadly defined, is a person who actively challenges an established doctrine, policy, or institution. When dissidents unite for a common cause they often affect a dissident movement.
The word has been used since 16th century in the context of religion. The noun was first used in the political sense in 1940, with the rise of such totalitarian systems as the Soviet Union.
The term dissident was used in the Eastern bloc, particularly in the Soviet Union, in the period following Joseph Stalin's death until the fall of communism. It was attached to citizens who criticized the practices or the authority of the Communist Party. The people who used to write and distribute non-censored, non-conformist samizdat literature were criticized in the official newspapers. Soon, many of those who were dissatisfied with the Soviet Bloc began to self-identify as dissidents. This radically changed the meaning of the term: instead of being used in reference to an individual who opposes society, it came to refer to an individual whose non-conformism was perceived to be for the good of a society. An important element of dissident activity in the USSR was informing society (both inside the Soviet Union and in foreign countries) about violation of laws and human rights: see Chronicle of Current Events and Moscow Helsinki Group. Some famous Soviet dissents were Aleksandr Solzhenitsyn and Andrei Sakharov.
The term dissident has become the primary term to describe Irish republicans who politically continue to oppose Good Friday Agreement of 1998 and reject the outcome of the referendums on it. These political parties also have paramilitary wings which espouse violent methods to achieve a United Ireland.