cotyloid joint vs diarthrosis

cotyloid joint and diarthrosis both are nouns.

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

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

cotyloid joint: a freely moving joint in which a sphere on the head of one bone fits into a rounded cavity in the other bone
diarthrosis: a joint so articulated as to move freely

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

cotyloid joint (noun)	diarthrosis (noun)
a freely moving joint in which a sphere on the head of one bone fits into a rounded cavity in the other bone	a joint so articulated as to move freely