ischium vs ischial bone

ischium and ischial bone both are nouns.

Word	Noun	Adjective	Verb	Adverb
ischium	Yes	No	No	No
ischial bone	Yes	No	No	No

As nouns, ischium and ischial bone are synonyms defined as:

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

Other synonyms of ischium include os ischii.

ischium (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