The ilium is the largest of three bones that fuse together over time to comprise the outer sections of the pelvis. It is present in most vertebrates, excluding bony fish and most snakes.

In humans, it is divided into two sections: the body and the ala, indicated by a line on the surface of the bone. The other two bones that form the fused pelvis are the ischium and the pubis, which lie below the ilium.

The widest measurement of the pelvis, taken round the upper iliac bones, is referred to as the biiliac width. This measurement is critically significant in obstetrics, as it can predict whether or not a pregnant woman will need a caesarean section, dependent on the size of the baby's head.

Together with the ischium and the pubis, the ilium is part of the acetabulum, a concave structure forming the socket of the ball-and-socket hip joint. The hollow created by the three bones holds the femur head, the uppermost part of the thigh bone.