The Collaborative International Dictionary of English v.0.48:

Appendix \Ap*pen"dix\, n.; pl. E. Appendixes, L. Appendices.
   [L. appendix, -dicis, fr. appendere. See Append.]
   1. Something appended or added; an appendage, adjunct, or
      concomitant.
      [1913 Webster]

            Normandy became an appendix to England. --Sir M.
                                                  Hale.
      [1913 Webster]

   2. Any literary matter added to a book, but not necessarily
      essential to its completeness, and thus distinguished from
      supplement, which is intended to supply deficiencies and
      correct inaccuracies.
      [1913 Webster]

   3. (Anatomy) The vermiform appendix.
      [Webster 1913 Suppl.]

   Syn: See Supplement.
        [1913 Webster]

WordNet (r) 3.0 (2006):

appendix
    n 1: supplementary material that is collected and appended at
         the back of a book
    2: a vestigial process that extends from the lower end of the
       cecum and that resembles a small pouch [syn: appendix,
       vermiform appendix, vermiform process, cecal appendage]

Moby Thesaurus II by Grady Ward, 1.0:

99 Moby Thesaurus words for "appendix":
   PS, Parthian shot, abdomen, accessory, addendum, adjunct, affix,
   afterthought, allonge, anus, appurtenance, back matter, blind gut,
   bowels, brain, cecum, chorus, coda, codicil, colon, colophon,
   commentary, conclusion, consequence, continuance, continuation,
   double take, duodenum, dying words, enclitic, endocardium,
   entrails, envoi, epilogue, follow-through, follow-up, foregut,
   giblets, gizzard, guts, heart, hindgut, infix, innards,
   inner mechanism, insides, interlineation, internals, interpolation,
   intestine, inwards, jejunum, kidney, kishkes, large intestine,
   last words, liver, liver and lights, lung, marginalia, midgut,
   note, parting shot, perineum, peroration, postface, postfix,
   postlude, postscript, prefix, proclitic, pump, pylorus, rectum,
   refrain, rider, scholia, second thought, sequel, sequela, sequelae,
   sequelant, sequent, sequitur, small intestine, spleen, stomach,
   subscript, suffix, supplement, swan song, tag, tail, ticker,
   tripes, vermiform appendix, viscera, vitals, works