What is the definition or description of: sacrum?

Lower back bone: Below the lumbar spine and above the tailbone or coccyx lies the sacrum. The sacrum is part of the spine and interacts with the pelvis at the sacro-iliac joints.

Base of spine.: The sacrum is a large flat trapezoidal bone, found at the base of the spine. It forms the posterior portion of the pelvis. There are four bones surrounding the sacrum: above is the lumbar vertebre (usually l5); below is the coccyx (tailbone) and to either side are the illiac bones of the pelvis. In front of the scarum is the pelvic cavity, and the posterior surface can be palpated through the skin.