An addendum is something added to the end of a book, document, or agreement to include extra information that wasn't in the original version. Think of it as an official “P.S.” that gets attached after the main part is finished.

When an author discovers a helpful fact after her book is already printed, she might include it in an addendum in the next edition. When two parties sign a contract but later need to adjust one detail, they can add an addendum rather than rewriting the entire document. Scientists sometimes publish an addendum to a research paper when new data becomes available.

Unlike a footnote (which goes at the bottom of a page) or an appendix (which contains supporting material like charts or lists), an addendum specifically adds new information that wasn't included when the original was created.

The plural can be either addendums or addenda. You might hear someone say, “We need to draft an addendum to clarify that point,” or “The contract includes three addenda covering different situations.”