1. The large heavy bone at the base of the spine, which is made up of the fused vertebrae: During her anatomy class, Heather was asked to locate the sacrum of the skeleton as part of the examination.
2. Etymology: "bone at the base of the spine", from Late Latin os sacrum, "sacred bone"; from Latin os, "bone" + sacrum, neuter of sacer, "sacred".

Said to be called a sacrum because this bone was the part of animals that was offered more often in sacrifices; a translation of Greek hieron osteon (use of the bone in sacrificial ceremonies).

The sacrum is located in the vertebral column, between the lumbar vertebrae (largest segments of the movable part of the vertebral column) and the coccyx (lower end of the vertebral column). It is roughly triangular in shape and makes up the back wall of the pelvis.

The female sacrum is normally wider and less curved than that of the male.