A threshold above a simple majority, such as two-thirds or three-fifths, required to adopt certain decisions.
In more detail
Supermajority rules protect constitutions and minorities by making change harder. Article V of the US Constitution requires two-thirds of both houses of Congress to propose amendments and ratification by three-fourths of the states. The cost is a bias towards the status quo, since a minority can block. Buchanan and Tullock argued the best threshold trades decision-making costs against the external costs imposed on losers.