The Collaborative International Dictionary of English v.0.48:

Midriff \Mid"riff\ (m[i^]d"r[i^]f), n. [AS. midhrif; midd mid,
   middle + hrif bowels, womb; akin to OFries. midref midriff,
   rif, ref, belly, OHG. href body, and to L. corpus body. See
   Corpse.] (Anat.)
   1. See Diaphragm, n., 2.
      [1913 Webster]

   2. The middle part of the front of the body, from the waist
      to the chest.
      [PJC]

            Smote him into the midriff with a stone. --Milton.
      [1913 Webster]

   3. That part of a garment, especially a dress or bodice, that
      covers the midriff[2].
      [PJC] Mid sea

WordNet (r) 3.0 (2006):

midriff
    n 1: the middle area of the human torso (usually in front);
         "young American women believe that a bare midriff is
         fashionable" [syn: middle, midriff, midsection]
    2: (anatomy) a muscular partition separating the abdominal and
       thoracic cavities; functions in respiration [syn:
       diaphragm, midriff]

Moby Thesaurus II by Grady Ward, 1.0:

83 Moby Thesaurus words for "midriff":
   abdomen, abomasum, barrier, bay window, beerbelly, belly, brattice,
   breadbasket, buffer, buffer state, bulkhead, bumper, center,
   cloison, collision mat, core, craw, crop, cushion, diameter,
   diaphragm, dissepiment, dividing line, dividing wall, division,
   embonpoint, equator, fender, first stomach, gizzard, gullet, gut,
   heart, honeycomb stomach, interior, interseptum, kernel, kishkes,
   manyplies, mat, maw, mean, median, middle, midmost, midsection,
   midst, nucleus, omasum, pad, panel, paries, partition, party wall,
   paunch, pot, potbelly, potgut, property line, psalterium, pusgut,
   rennet bag, reticulum, rumen, second stomach, separation, septulum,
   septum, shock pad, spare tire, stomach, swagbelly, thick,
   thick of things, third stomach, tum-tum, tummy, underbelly,
   ventripotence, waist, waistline, wall, zone