The Collaborative International Dictionary of English v.0.48:
Codicil \Cod"i*cil\, n. [L. codicillus, dim. of codex: cf. F. codicille. See Code.] (Law) A clause added to a will. [1913 Webster]WordNet (r) 3.0 (2006):
codicil n 1: a supplement to a will; a testamentary instrument intended to alter an already executed will