The Collaborative International Dictionary of English v.0.48:

Supplement \Sup"ple*ment\, n. [F. suppl['e]ment, L.
   supplementum, fr. supplere to fill up. See Supply, v. t.]
   1. That which supplies a deficiency, or meets a want; a
      store; a supply. [Obs.] --Chapman.
      [1913 Webster]

   2. That which fills up, completes, or makes an addition to,
      something already organized, arranged, or set apart;
      specifically, a part added to, or issued as a continuation
      of, a book or paper, to make good its deficiencies or
      correct its errors.
      [1913 Webster]

   3. (Trig.) The number of degrees which, if added to a
      specified arc, make it 180[deg]; the quantity by which an
      arc or an angle falls short of 180 degrees, or an arc
      falls short of a semicircle.
      [1913 Webster]

   Syn: Appendix.

   Usage: Appendix, Supplement. An appendix is that which is
          appended to something, but is not essential to its
          completeness; a supplement is that which supplements,
          or serves to complete or make perfect, that to which
          it is added.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Supplement \Sup"ple*ment\, v. t. [imp. & p. p. Supplemented;
   p. pr. & vb. n. Supplementing.]
   To fill up or supply by addition; to add something to.
   [1913 Webster]

         Causes of one kind must be supplemented by bringing to
         bear upon them a causation of another kind. --I.
                                                  Taylor.
   [1913 Webster] Supplemental

WordNet (r) 3.0 (2006):

supplement
    n 1: textual matter that is added onto a publication; usually at
         the end [syn: addendum, supplement, postscript]
    2: a quantity added (e.g. to make up for a deficiency) [syn:
       supplement, supplementation]
    3: a supplementary component that improves capability [syn:
       accessory, appurtenance, supplement, add-on]
    v 1: add as a supplement to what seems insufficient; "supplement
         your diet"
    2: serve as a supplement to; "Vitamins supplemented his meager
       diet"
    3: add to the very end; "He appended a glossary to his novel
       where he used an invented language" [syn: append, add on,
       supplement, affix]

Moby Thesaurus II by Grady Ward, 1.0:

102 Moby Thesaurus words for "supplement":
   PS, Parthian shot, accession, accessory, accident, accidental,
   accompaniment, add to, addenda, addendum, additament, addition,
   additive, additory, additum, adjunct, adjuvant, afterthought,
   annex, annexation, appanage, appendage, appendant, appendix,
   appurtenance, appurtenant, attachment, augment, augmentation,
   auxiliary, back matter, chorus, coda, codicil, collateral,
   colophon, complement, conclusion, concomitant, consequence,
   contingency, contingent, continuance, continuation, corollary,
   double take, dying words, envoi, epilogue, extend, extension,
   extra, extrapolation, fixture, follow-through, follow-up, fortify,
   happenstance, incidental, increase, increment, inessential, insert,
   last words, mere chance, nonessential, not-self, offshoot, other,
   parting shot, pendant, peroration, postface, postfix, postlude,
   postscript, recruit, refrain, reinforce, reinforcement, rider,
   second thought, secondary, sequel, sequela, sequelae, sequelant,
   sequent, sequitur, side effect, side issue, strengthen, subscript,
   subsidiary, suffix, superaddition, supplementation, swan song, tag,
   tailpiece, undergirding, unessential