Sacrum: The large heavy bone at the base of the spine, which is made up of fused sacral vertebrae. The sacrum is located in the vertebral column, between the lumbar vertebrae and the coccyx. It is roughly triangular in shape and makes up the back wall of the pelvis. The female sacrum is wider and less curved than the male.
The name comes from the Latin "os sacrum" (holy bone). Why it was sacred is matter of conjecture.