trapezium vs os trapezium

trapezium and os trapezium both are nouns.

Word	Noun	Adjective	Verb	Adverb
trapezium	Yes	No	No	No
os trapezium	Yes	No	No	No

As nouns, trapezium and os trapezium are synonyms defined as:

trapezium and os trapezium: the wrist bone on the thumb side of the hand that articulates with the 1st and 2nd metacarpals

Other synonyms of trapezium include trapezium bone.

trapezium (noun)	os trapezium (noun)
the wrist bone on the thumb side of the hand that articulates with the 1st and 2nd metacarpals	the wrist bone on the thumb side of the hand that articulates with the 1st and 2nd metacarpals
a quadrilateral with no parallel sides