ischial bone vs os ischii

ischial bone and os ischii both are nouns.

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

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

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

Other synonyms of ischial bone include ischium.

ischial bone (noun)	os ischii (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