John Stuart Mill begins his book On Liberty with an account of what he calls "The Tyranny of the Majority." He means "majority rule" and it refers to the power that a voting majority of people in a sovereign state have over a minority. It was once thought that majority rule is a good thing, the backbone of democracy. But Mill says, a majority can vote to suppress the liberty of a minority, as happened in the U.S.A. during the many years of slavery, and as continued to happen after the Civil War during the time of Jim Crow when a white majority made it impossible for a black minority to vote or to get a decent education.
The question that Mill asks is normative. Under what conditions is it morally right for a majority to suppress the liberty of a minority?