Liberum veto (Latin: "free veto") was a parliamentary device in the Polish-Lithuanian Commonwealth that allowed any deputy to a Sejm to force an immediate end to the current session and nullify all legislation already passed at it.
This rule evolved from a unanimity principle, and the latter from the federative character of the Polish-Lithuanian Commonwealth, which was essentially a federation of countries. Each deputy to a Sejm was elected at a local regional sejm (sejmik) and represented the entire region. He thus assumed responsibility to his sejmik for all decisions taken at the Sejm. A decision taken by a majority against the will of a minority (even if only a single sejmik) was considered a violation of the principle of political equality.
It is commonly, and erroneously, believed that a Sejm was first disrupted by means of liberum veto by a Trakai deputy, Władysław Siciński, in 1652. In reality, however, he only vetoed the continuation of the Sejm's deliberations beyond the statutory time limit. It was only in 1669, in Krakow, that a Sejm was prematurely disrupted on the strength of the liberum veto, by the Kyiv deputy, Adam Olizar.
In the first half of the 18th century, it became increasingly common for Sejm sessions to be broken up by liberum veto, as the Commonwealth's neighbors — chiefly Russia and Prussia — found this a useful tool to frustrate attempts at reforming and strengthening the Commonwealth. The latter deteriorated from a European power into a state of anarchy.
After 1764 the liberum veto practically went out of use: the principle of unanimity did not bind "confederated sejms," and so deputies formed a "confederation" (Polish: konfederacja) at the beginning of a session in order to prevent its disruption by liberum veto.
The liberum veto was abolished by the May 3rd, 1791, Constitution (adopted by a confederated sejm), which permanently established the principle of majority rule.
The achievements of that constitution, however — Europe's first written national constitution — were undone by another confederated sejm, meeting at Grodno in 1793. That Sejm, under duress from Russia and Prussia, ratified the penultimate, Second Partition of the Polish-Lithuanian Commonwealth.