Sacrum is defined as the large heavy bone at the base of the spine, made up of fused sacral vertebrae. In humans, the sacrum is located in the vertebral column, between the lumbar vertebrae and the coccyx.
Sacrum is roughly triangular in shape and makes up the back wall of the pelvis. If you must know, the female sacrum is wider and less curved than that of male.
This medical dictionary page, provides definitions for the medical term ‘Sacrum‘, bone of the buttocks.