os ischii vs ischial bone

os ischii and ischial bone both are nouns.

Word	Noun	Adjective	Verb	Adverb
os ischii	Yes	No	No	No
ischial bone	Yes	No	No	No

As nouns, os ischii and ischial bone are synonyms defined as:

os ischii and ischial bone: one of the three sections of the hipbone; situated below the ilium

Other synonyms of os ischii include ischium.

os ischii (noun)	ischial bone (noun)
one of the three sections of the hipbone; situated below the ilium	one of the three sections of the hipbone; situated below the ilium