in government and law, an addition or alteration made to a constitution, statute, or legislative bill or resolution. Amendments can be made to existing constitutions and statutes and are also commonly made to bills in the course of their passage through a legislature. Since amendments to a national constitution can fundamentally change a country's political system or governing institutions, such amendments are usually submitted to an exactly prescribed procedure.

The best-known amendments are those that have been made to the U.S. Constitution; Article V makes provision for the amendment of that document. The first 10 amendments that were made to the Constitution are called the Bill of Rights. (See Rights, Bill of.) A total of 27 amendments have been made to the Constitution. For an amendment to be made, two-thirds of the members of each house of Congress must approve it, and three-fourths of the states must ratify it. Congress decides whether the ratification will be by state legislatures or by popularly elected conventions in the several states (though in only one instance, that of the Twenty-First Amendment, which repealed prohibition, was the convention system used). In many U.S. states, proposed amendments to a state constitution must be approved by the voters in a popular referendum.