Simple Definition:

An amendment is a change or addition made to a document or law [a written set of rules or guidelines] that modifies or updates its content. It is a formal alteration that is made to improve, correct, or expand upon the original text. For example, the United States Constitution has 27 amendments that have been added since it was first written in 1787. One famous amendment is the First Amendment, which guarantees freedom of speech, religion, and the press.

Very Simple Definition:

An amendment is a change made to a document or law. For example, if you have a rule that says you cannot eat dairy products, but you don’t have anything else to eat, an amendment could be made to allow you to eat dairy products in small amounts.