A formal amendment to the U.S. Constitution is one that goes through one of the processes outlined in Article V of the Constitution and adds to or changes the document's wording. In contrast, an informal amendment does not include a change to the wording of the Constitution but reflects a change in how the people or the courts interpret its meaning.
Article V of the Constitution establishes two ways in which an amendment can be both proposed and ratified. The U.S. National Archives outlines the specific procedures by which an amendment becomes a formal amendment. In the first method of proposing an amendment, two-thirds of both houses of Congress adopt a proposed amendment. The alternative is for two-thirds of the state legislatures to request that Congress call a Constitutional Convention; this method has never been used. To ratify a proposed amendment, three-fourths of the state legislatures must ratify it or ratifying conventions in three-fourths of the states must approve it.
The large majorities needed to ratify proposed amendments makes it extremely difficult to pass formal amendments, according to the University of Missouri–Kansas City School of Law. Because amendments are changes to the supreme law of the land, a formal amendment is the ultimate legislative check against the executive veto and Supreme Court's judicial review.