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

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "codicil":
   PS, Parthian shot, addendum, affix, afterthought, allonge,
   appendix, attested copy, back matter, bequeathal, bequest, chorus,
   coda, colophon, commentary, conclusion, consequence, continuance,
   continuation, devise, double take, dying words, enclitic, envoi,
   epilogue, follow-through, follow-up, infix, inheritance,
   interlineation, interpolation, last words, legacy, marginalia,
   note, parting shot, peroration, postface, postfix, postlude,
   postscript, prefix, probate, proclitic, refrain, rider, scholia,
   second thought, sequel, sequela, sequelae, sequelant, sequent,
   sequitur, subscript, suffix, supplement, swan song, tag, tail,
   testament, will



Bouvier's Law Dictionary, Revised 6th Ed (1856):

CODICIL, devises. 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. 4 Bro. C. C. 55; 2 Ves. sen. 242 4 Ves.
610; 2 Ridgw. Irish P. C. 11, 43.
     2. 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. 8 Cowen, Rep. 56.,
     3. Formerly, the difference between a will and a codicil consisted in
this, that in the former an executor was named, while in the latter none was
appointed. Swinb. part 1, s. 5, pl. 2; Godolph. Leg. part 1, c. 6, s. 2.
This is the distinction of the civil law, and adopted by the canon law. Vide
Williams on Wills, ch. 2; Rob. on Wills, 154, n. 388, 476; Lovelass on
Wills, 185, 289 4 Kent, Com. 516; 1 Ves. jr. 407, 497; 3 Ves. jr. 110; 4
Ves. jr. 610; 1 Supp. to Ves. jr. 116, 140.
     4. 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.
     5. 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. Just. 2, 25; Bowy. Com. 155, 156.
     6. The form of devising by codicil is abolished in Louisiana; Code,
1563; and whether the disposition of the property be made by testament,
under this title, or under that of institution of heir, of legacy, codicil,
donation mortis causa, or under any other name indicating the last will,
provided it be clothed with the forms required for the validity of a
testament, it is, as far as form is concerned, to be considered a testament.
Ib. Vide 1 Brown's Civil Law, 292; Domat, Lois Civ. liv. 4, t. 1, s. 1;
Lecons Element, du Dr. Civ. Rom. tit. 25.