An addition or modification to an existing will is referred to as what?

The correct term for an addition or modification to an existing will is a codicil. A codicil allows the testator (the person who made the will) to make changes or updates without needing to create an entirely new will. This could involve changes to beneficiaries, alterations in how property is distributed, or updates to executors. Codicils must meet the same legal requirements as the original will, typically requiring the same formalities such as signature and witness presence.

In contrast, a testament refers more generally to a will itself or a document that indicates the wishes regarding the distribution of property upon death and is not specifically for amendments. An affidavit is a written statement confirmed by oath, used in legal proceedings and is unrelated to the modification of wills. A declaration, while it can be a formal statement of fact, does not specifically pertain to changes in wills or estate planning.