A joint so articulated as to move freely WordNet 3.0 & Open English WordNet

A synovial joint, also known as diarthrosis, joins bones or cartilage with a fibrous joint capsule that is continuous with the periosteum of the joined bones, constitutes the outer boundary of a synovial cavity, and surrounds the bones' articulating surfaces. Wikipedia

An articulation which admits of free motion in the joint; the most common type of articulation Wikidata

(Anatomy) An articulation which admits of free motion in the joint; the most common type of articulation. OmegaWiki

A joint that can move freely in various planes. Wiktionary