Os ilium

The ilium (plural ilia) is the uppermost and largest part of the hip bone, and appears in most vertebrates including mammals and birds, but not bony fish.

All reptiles have an ilium except snakes, although some snake species have a tiny bone which is considered to be an ilium.

The ilium of the human is divisible into two parts, the body and the wing; the separation is indicated on the top surface by a curved line, the arcuate line, and on the external surface by the margin of the acetabulum.

The name comes from the Latin (ile, ilis), meaning "groin" or "flank".