Source : Webster's Revised Unabridged Dictionary (1913)

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

   2. (Zo["o]l.) One of the pleur[ae] of insects.

Source : WordNet®

ischium
     n : one of the three sections of the hipbone; situated below the
         ilium [syn: {ischial bone}, {os ischii}]
     [also: {ischia} (pl)]