Definition of CODICIL: (noun) / a supplement to a will
New York Estates Powers & Trusts Law defines a codicil as a supplement to a will, either adding to, taking from or altering its provisions or confirming it in whole or in part by republication, but not totally revoking such will (EPTL § 1-2.1).
Plain English translation: a codicil is similar to an addendum or rider to a contract; it typically changes only some of the terms included in the will.
Codicils have become rare since the advent of modern computers and word processing templates. In the pre-digital age, a codicil was a shortcut used to avoid having to retype an entire will. Today, attorneys often save the final draft of their clients' wills in a word processing program and can easily make changes. Even without the final draft, a new will can be populated relatively quickly.