The Collaborative International Dictionary of English v.0.48:

Ileum \Il"e*um\, n. [L. ile, ileum, ilium, pl. ilia, groin,
   flank.]
   1. (Anat.) The last, and usually the longest, division of the
      small intestine; the part between the jejunum and large
      intestine. [Written also ileon, and ilium.]
      [1913 Webster]

   2. (Anat.) See Ilium. [R.]
      [1913 Webster]

   Note: Most modern writers restrict ileum to the division of
         the intestine and ilium to the pelvic bone.
         [1913 Webster]