addendum

An addendum (plural: addenda) is a supplementary piece added to a document—essentially an item of additional material. It is used when you need to clarify or expand upon something that already exists without completely rewriting the original text.

Unlike a revision, which changes existing content, an addendum adds new information. This makes it particularly useful in formal settings like legal agreements, contracts, and academic papers. For example, a rental agreement might have an addendum detailing pet policies or parking arrangements. A scientific report could include an addendum to present newly discovered data that arrived after the report was finished.

The term "addenda" is the standard plural form (though "addendums" also appears in everyday use). Unlike an appendix, which is usually planned as part of the original document, an addendum is typically created after the fact. In legal and contractual contexts, it generally needs to be clearly referenced, attached, and properly approved (often signed) to carry full force. Think of it as a necessary extension, carefully attached to ensure completeness and accuracy. While you might append something casually, an addendum suggests a more formal and deliberate addition.