sacrum
Concise Oxford English Dictionary © 2008 Oxford University Press:
sacrum /ˈseɪkrəm, ˈsak-/
▶noun (pl. sacra /-krə/ or sacrums) Anatomy a triangular bone in the lower back formed from fused vertebrae and situated between the two hip bones of the pelvis.
– origin C18: from L. os sacrum, translation of Gk hieron osteon ‘sacred bone’ (from the belief that the soul resides in it).
'sacrum' also found in these Oxford entries:

