Addendum Definition: An attachment to a written document. Related Terms: Treaty In Trent v. PPG Industries, Justice Lanchard of the Court of Appeal of Louisiana wrote:"An addendum is something which is added, such as a section of added material. Thus, by its very nature, it would neither be numbered sequentially nor, necessarily, appear before other attachments to a contract." The Australian Legal Dictionary definition is helpful though focusing on international law:"Something to be added."An addition to a treaty negotiated after signing of the treaty. An addendum to a treaty must be ratified separately by a member state of the treaty; ratification of the original treaty does not imply ratification of the addendum."REFERENCES:Nygh, Peter and Butt, P., editors, Australian Legal Dictionary (Sydney: Butterworths, 1997), page 25.Trent v. PPG Industries, Inc., 865 So. 2d 1041 (2004) Categories & Topics: Civil Litigation & Evidence Law Dictionary International Law Dictionary Find you are constantly looking up definitions? Try our search provider (works in most modern browsers) If you find an error or omission in Duhaime's Legal Dictionary, or if you have legal term suggestion, we'd love to hear from you!