Legal Definition of Codicils: An addition or supplement to a will. It may also delete or modify various provisions of a will. (Codicils must be witnessed and signed in the same manner as the original will.) | Looking to modify your will? Learn about the importance and versatility of codicils in estate planning. Discover how these simple additions can save you time and expenses while ensuring your final wishes are legally binding. Consult with a qualified attorney for expert guidance