Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

sacrum
     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)]

sacra
     See {sacrum}