ilium vs pelvic arch

ilium and pelvic arch both are nouns.

Word	Noun	Adjective	Verb	Adverb
ilium	Yes	No	No	No
pelvic arch	Yes	No	No	No

ilium (noun)	pelvic arch (noun)
the upper and widest of the three bones making up the hipbone	the structure of the vertebrate skeleton supporting the lower limbs in humans and the hind limbs or corresponding parts in other vertebrates