An addendum is an extra piece you put on after the main text is already there — at the end of a book, a report, a speech, or, very often, a contract. It adds information, or it changes something, without rewriting the whole thing. In books, people also say appendix or supplement. Addendum often feels like “we needed to add this later,” after the first version had gone out. In legal English it is quite at home: a signed extra page that tweaks the deal. It is a formal, paper word. Though actually, almost nobody giving a speech says “I have an addendum.” They say “one more thing” or “let me add.” You meet this word on the page. The old Latin plural is addenda, and careful writers still use it; addendums appears too, and some readers wince. One addendum, two addenda is the tidy pattern. It is not a random extra paragraph in the middle; the idea is “stuck on, usually at the end, because the main thing was already done.”