fibrous joint vs articulatio

fibrous joint and articulatio both are nouns.

Word	Noun	Adjective	Verb	Adverb
fibrous joint	Yes	No	No	No
articulatio	Yes	No	No	No

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

fibrous joint: an immovable joint (especially between the bones of the skull)
articulatio: (anatomy) the point of connection between two bones or elements of a skeleton (especially if it allows motion)

Other hypernyms of fibrous joint include articulation, joint.

fibrous joint (noun)	articulatio (noun)
an immovable joint (especially between the bones of the skull)	(anatomy) the point of connection between two bones or elements of a skeleton (especially if it allows motion)