Sa·crum n.; pl. sacra Anat. That part of the vertebral column which is directly connected with, or forms a part of, the pelvis.
Note: ☞ It may consist of a single vertebra or of several more or less consolidated. In man it forms the dorsal, or posterior, wall of the pelvis, and consists of five united vertebrae, which diminish in size very rapidly to the posterior extremity, which bears the coccyx.
n : wedge-shaped bone consisting of five fused vertebrae forming
the posterior part of the pelvis; its base connects with
the lowest lumbar vertebra and its tip with the coccyx
[also: sacra (pl)]