The liberum veto (Latin for "free veto") was a parliamentary device in the Polish–Lithuanian Commonwealth. It was a form of unanimity voting rule that allowed any member of the Sejm (legislature) to force an immediate end to the current session and to nullify any legislation that had already been passed at the session by shouting either Sisto activitatem! (Latin: "I stop the activity!") or Nie pozwalam! (Polish: "I do not allow!"). The rule was in place from the mid-17th century to the late 18th century in the Sejm's parliamentary deliberations. It was based on the premise that since all Polish noblemen were equal, every measure that came before the Sejm had to be passed unanimously. The liberum veto was a key part of the political system of the Commonwealth, strengthening democratic elements and checking royal power and went against the European-wide trend of having a strong executive (absolute monarchy).

Many historians hold that the liberum veto was a major cause of the deterioration of the Commonwealth political system, particularly in the 18th century, when foreign powers bribed Sejm members to paralyze its proceedings, and the Commonwealth's eventual destruction in the partitions of Poland and foreign occupation, dominance and manipulation of Poland. Piotr Stefan Wandycz wrote that the "liberum veto had become the sinister symbol of old Polish anarchy". In the period of 1573–1763, about 150 sejms were held, about a third failing to pass any legislation, mostly because of the liberum veto. The expression Polish parliament in many European languages originated from the apparent paralysis.
How was Liberum Veto mis-used?
With Liberum veto,  it was only possible to pass any legislation with unanimous consent. So one member of parliament to stop any legislation. Adversaries would often influence or bribe just one member of Parliament to derail a legislation.