The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   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.
         [1913 Webster]

WordNet (r) 3.0 (2006):

sacrum
    n 1: 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