The right of any single member of the Polish Sejm to block a measure or end a session, so that legislation in practice required unanimity.
In more detail
First used to end a Sejm session in 1652 by Władysław Siciński, it later became a routine means of obstruction and foreign influence. 'Confederated' Sejms were exempt from it, and the Constitution of 3 May 1791 abolished it. Scholars note that it originated as a consensus principle among nobles regarded as equals before degenerating into a tool of paralysis.