ischium vs os ischii

ischium and os ischii both are nouns.

Word	Noun	Adjective	Verb	Adverb
ischium	Yes	No	No	No
os ischii	Yes	No	No	No

As nouns, ischium and os ischii are synonyms defined as:

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

Other synonyms of ischium include ischial bone.

ischium (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