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]