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An addendum is an addition to a finished document, such as a contract. The most common addendum is an attachment or exhibit at the end of such a document.  For example, a contract to manufacture widgets may have an addendum listing the specifications for said widgets. As this modifies the original document, it should be signed or executed with the same formalities, unless the original document states otherwise.

An addendum should not be confused with an amendment of a contract, as an addendum does not modify substantial terms of the document. 

On the other hand, in the civil law systems, addendum means any modification of a contract (for example an extension of the terms, change in payment currency, new collaterals), not just inclusion of additional attachments or exhibits.

[Last updated in January of 2022 by the Wex Definitions Team]