Source : Webster's Revised Unabridged Dictionary (1913)

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.

   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.

   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.

   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.

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.

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

Source : WordNet®

supplement
     n 1: textual matter that is added onto a publication; usually at
          the end [syn: {addendum}, {postscript}]
     2: a quantity added (e.g. to make up for a deficiency) [syn: {supplementation}]
     3: a supplementary component that improves capability [syn: {accessory},
         {appurtenance}, {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}, {affix}]