Tyranny of the majority
"Tyranny of the majority" refers to the problem in any democratic society wherein the majority are able to exploit one or more minority groups in some way and where the system gives this exploitation the appearance of legitimacy because it was done via legitimate means (i.e. within the system).
The obvious example is slavery, which was legal in the United States until the Emancipation Proclamation of January 1, 1863. The white majority made laws, via rules emerging from a nominally democratic process, which allowed white people to own black people.
The risk of such tyranny was pointed out by US cofounder James Madison in Federalist #10, although he did not actually use the phrase:
Complaints are everywhere heard from our most considerate and virtuous citizens, equally the friends of public and private faith, and of public and personal liberty, that our Governments are too unstable; that the public good is disregarded in the conflicts of rival parties; and that measures are too often decided, not according to the rules of justice, and the rights of the minor party, but by the superior force of an interested and overbearing majority.