os ischii vs ischium

os ischii and ischium both are nouns.

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

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

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

Other synonyms of os ischii include ischial bone.

os ischii (noun)	ischium (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