Codicil
A separate document that amends or adds to an existing will without replacing it entirely. A codicil must be signed and witnessed with the same formalities as a will. Today, most attorneys recommend creating a new will instead.
Legal Definition
A testamentary instrument executed with the same formalities as a will that modifies, supplements, or partially revokes an existing will without replacing it in its entirety.
Practical Example
After his will was signed, Fred wants to add his new grandchild as a beneficiary. His lawyer prepares a codicil that adds the grandchild and specifies their share, while leaving everything else in the original will unchanged.