The Collaborative International Dictionary of English v.0.48:

Ischion \Is"chi*on\, Ischium \Is"chi*um\, n. [L., Gr.
   'ischi`on.]
   1. (Anat.) The ventral and posterior of the three principal
      bones composing either half of the pelvis; seat bone; the
      huckle bone.
      [1913 Webster]

   2. (Zool.) One of the pleur[ae] of insects.
      [1913 Webster]

WordNet (r) 3.0 (2006):

ischium
    n 1: one of the three sections of the hipbone; situated below
         the ilium [syn: ischium, ischial bone, os ischii]