ilium
Concise Oxford English Dictionary © 2008 Oxford University Press:
ilium /ˈɪlɪəm/
▶noun (pl. ilia) the large broad bone forming the upper part of each half of the pelvis.
– origin ME (orig. denoting the ileum): from L., sing. of ilia ‘flanks, entrails’.
'ilium' also found in these Oxford entries:

