What is the X in X-mas?
ilium il·i·um (ĭl'ē-əm)
n. pl. il·i·a (-ē-ə)
The uppermost and widest of the three bones constituting either of the lateral halves of the pelvis. Also called iliac bone.
The uppermost and widest of the three bones that fuse together to form each of the hipbones. See more at skeleton.