A supplement or addition to a will that explains, modifies, or revokes a previous will provision or that adds an additional provision. A codicil must be signed and witnessed with the same formalities as those used in the will's preparation.

An addition or supplement to a will; it must be executed with the same solemnities. A codicil is a part of the will, the two instruments making but one will.

There may be several codicils to one will and the whole will be taken as one: the codicil does not consequently revoke the will further than it is in opposition to some of its particular dispositions, unless there be express words of revocation.

Formerly, the difference between a will and a codicil consisted in that in the former an executor was named while in the latter none was appointed. This is the distinction of the civil law and adopted by the canon law.

Codicils were chiefly intended to mitigate the strictness of the ancient Roman law, which required that a will should be attested by seven Roman citizens, omni exceptione majores. A legacy could be bequeathed, but the heir could not be appointed by codicil, though he might be made heir indirectly by way of fidei commissum.

Codicils owe their origin to the following circumstances. Lucius Lentulus, dying in Africa, left codicils confirmed by anticipation in a will of former date and in those codicils requested the emperor Augustus, by way of fidei commissum or trust, to do something therein expressed. The emperor carried this will into effect and the daughter of Lentulus paid legacies which she would not otherwise have been legally bound to pay. Other persons made similar fidei-commissa and then the emperor, by the advice of learned men whom he consulted, sanctioned the making of codicils and thus they became clothed with legal authority.