Sacrum Sa"crum, n.; pl. sacra. [NL., fr. L. sacer sacred, os sacrum the lowest bone of the spine.] (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 vertebr[ae], which diminish in size very rapidly to the posterior extremity, which bears the coccyx.