The medical term for the hip bone is "os coxa," or the innominate bone. The plural form is "os coxae." These large, flattened bones form the sides of the pelvis, one on each side of the sacrum.
The large innominate bones form the sides and the front of the pelvic cavity. They are made up of three fused bones: the ilium, the ischium and the pubis. In young children, these three bones are distinct from each other, but in adults, they fuse together to form one continuous bone. The fusing occurs around a cup-shaped articular cavity called the acetabulum. The innominate bones also connect with the femur bones in the legs.