sacrum

sacrum The large heavy bone at the base of the spine that is made up of the fused sacral vertebrae. In the vertebral column, the sacrum is situated between the lumbar vertebrae and the coccyx (tailbone). It is triangular in shape and forms the back wall of the pelvis. The female sacrum is wider and less curved than the male sacrum. From the Latin os sacrum, meaning “sacred bone” because it was used in sacrifice. See also pelvis; sacral vertebrae; vertebral column.