rotary joint vs diarthrosis

rotary joint and diarthrosis both are nouns.

Word	Noun	Adjective	Verb	Adverb
rotary joint	Yes	No	No	No
diarthrosis	Yes	No	No	No

As nouns, diarthrosis is a hypernym of rotary joint; that is, diarthrosis is a word with a broader meaning than rotary joint:

rotary joint: a freely moving joint in which movement is limited to rotation
diarthrosis: a joint so articulated as to move freely

Other hypernyms of rotary joint include articulatio synovialis, synovial joint.

rotary joint (noun)	diarthrosis (noun)
a freely moving joint in which movement is limited to rotation	a joint so articulated as to move freely