A "Formal Amendment" is one that actually adds to or changes the US Constitution. It is ratified by the states and becomes law.
An "informal amendment"is a change to the meaning or interpretation of the Constitution of the United States. There is no real informal way to change the Constitution, and it's not an actual change to the wording of the Constitution; rather, it's the way we perceive the Constitution that changes.
The one that comes to mind is the equal rights amendment (1972) - which prohibits the inequality of men and women. Opponents say that the amendment is no longer needed, as the issues are already law.