The Collaborative International Dictionary of English v.0.48:

Gizzard \Giz"zard\, n. [F. g['e]sier, L. gigeria, pl., the
   cooked entrails of poultry. Cf. Gigerium.]
   [1913 Webster]
   1. (Anat.) The second, or true, muscular stomach of birds, in
      which the food is crushed and ground, after being softened
      in the glandular stomach (crop), or lower part of the
      esophagus; the gigerium.
      [1913 Webster]

   2. (Zool.)
      (a) A thick muscular stomach found in many invertebrate
          animals.
      (b) A stomach armed with chitinous or shelly plates or
          teeth, as in certain insects and mollusks.
          [1913 Webster]

   Gizzard shad (Zool.), an American herring (Dorosoma
      cepedianum) resembling the shad, but of little value.

   To fret the gizzard, to harass; to vex one's self; to
      worry. [Low] --Hudibras.

   To stick in one's gizzard, to be difficult of digestion; to
      be offensive. [Low]
      [1913 Webster]

WordNet (r) 3.0 (2006):

gizzard
    n 1: thick-walled muscular pouch below the crop in many birds
         and reptiles for grinding food [syn: gizzard,
         ventriculus, gastric mill]

Moby Thesaurus II by Grady Ward, 1.0:

87 Moby Thesaurus words for "gizzard":
   abatis, abdomen, abomasum, anus, appendix, bay window, beerbelly,
   belly, blind gut, bowels, brain, brains, breadbasket, cecum,
   chitterlings, cockscomb, colon, craw, crop, diaphragm, duodenum,
   embonpoint, endocardium, entrails, first stomach, foregut, giblets,
   gullet, gut, guts, haslet, heart, hindgut, honeycomb stomach,
   innards, inner mechanism, insides, internals, intestine, inwards,
   jejunum, kidney, kidneys, kishkes, large intestine, liver,
   liver and lights, lung, manyplies, marrow, maw, midgut, midriff,
   omasum, paunch, perineum, pot, potbelly, potgut, psalterium, pump,
   pusgut, pylorus, rectum, rennet bag, reticulum, rumen,
   second stomach, small intestine, spare tire, spleen, stomach,
   swagbelly, sweetbread, third stomach, ticker, tongue, tripe,
   tripes, tum-tum, tummy, underbelly, ventripotence,
   vermiform appendix, viscera, vitals, works