An informal amendment process is used to refer to a method of changing the content of the U.S. Constitution in a way that isn't explicitly stated in the Constitution. While the Constitution itself outlines two specific ways to add amendments formally, "informal amendment" is a term often used to describe changes in interpretation of the Constitution due to societal changes or judicial review, according to Boundless.
According to USConstitution.net, the Constitution offers two fundamental ways to alter the document. A bill can be passed by a majority in both the Senate and the House of Representative and then passed onto the states for ratification, or the states themselves can call a Constitutional Convention where they can propose amendments. In both methods, a majority of state legislatures need to agree to pass the amendment.
USConstitution.net also points out two methods of informal amendment. The first occurs when the interpretation of the Constitution shifts due to changing societal circumstances. One example of this can be seen in the way voting rights have been extended over the centuries from just landowners to anyone over 18 years of age.
The other informal amendment method involves judicial review. Because the judicial branch is in charge of interpreting the Constitution, any changes in the judicial view of the Constitution change the way that the Constitution works in society, according to USConstitution.net.
USConstitution.net also reports on a third possible method of amendment, the "popular amendment," a method which has never been used. James Wilson proposed this idea in 1787, stating that the people of the United States should be able to change the Constitution when and how they please. No process exists, however, to put this method into practice.