The sacrum in human anatomy is a large, triangular bone at the base of the spine that forms by the fusing of the sacral vertebrae between ages 18 and 30. The sacrum situates at the upper, back part of the pelvic cavity, between the two wings of the pelvis. It forms joints with four other bones. It is a complex structure providing support for the spine and accommodation for the spinal nerves. It also articulates with the hip bones.

Source: Wikipedia