Short answer. A codicil is a supplement or addition to a will, made after the will was executed and annexed to it as part of it, used to explain, add to, or alter what the original will disposed of. Article 825 defines it this way but does not itself set out its formalities.
What the law says
A codicil is supplement or addition to a will, made after the execution of a will and annexed to be taken as a part thereof, by which disposition made in the original will is explained, added to, or altered.
Civil Code, Article 825 — Codicil Defined. Read the full provision →
A codicil is an addition, not a new will
Article 825 defines a codicil as a supplement or addition to a will, made after the execution of a will and annexed to be taken as a part thereof. Two things stand out in that wording. First, timing: a codicil comes after the original will already exists. Second, relationship: it is annexed to the will and taken as part of it, not treated as a separate, standalone document. A codicil does not replace the will — it attaches to it and becomes one of its parts.
What a codicil is used to do
The article describes the codicil's function directly: it is what disposition made in the original will is explained, added to, or altered through. That covers three distinct kinds of change to a will's original dispositions — clarifying language that was unclear, adding something the will did not originally cover, or changing something the will already provided for. A codicil is the vehicle for any of these, applied to dispositions the testator already made in the will it is annexed to.
What Article 825 does not tell you
The definition in Article 825 says what a codicil is and what it is for — it does not itself set out the formalities a codicil has to satisfy, such as whether it needs to be notarized, witnessed, or otherwise executed in a particular way. Anyone preparing or relying on a codicil needs to look to the rules on how a will is validly executed, since a codicil is, by this article's own definition, taken as part of the will it supplements.
Why the distinction matters in practice
Because a codicil is annexed to the will and treated as part of it, rather than existing on its own, its purpose is narrower than making a fresh will: it is for explaining, adding to, or altering dispositions the testator already made. Someone who wants to change a will substantially, rather than adjust or clarify what is already there, should be clear about which of those two things they are actually doing before assuming a codicil is the right instrument for it.