A testamentary document which is signed in the same way as a Will A codicil supplements the terms of an existing Will, either by adding to it, amending it or revoking part of it
A change to an existing will A supplement, amendment, or addition to a will that explains, modifies, adds to, subtracts from, qualifies, alters, restrains or revokes provisions in a will It becomes a part of the will and must be executed in the same manner as the will, as prescribed by state law
A codicil is an instruction that is added to a will after the main part of it has been written. a document making a change or addition to a will (=a legal document saying who you want your money and property to go to when you die) (codicillus, from codex; CODEX)