The Collaborative International Dictionary of English v.0.48:

Diarthrosis \Di`ar*thro"sis\, n. [NL., fr. Gr. ?, fr. ? to
   joint, articulate; dia` through, asunder + ? to fasten by a
   joint, 'a`rqron joint.] (Anat.)
   A form of articulation which admits of considerable motion; a
   complete joint; abarticulation. See Articulation.
   [1913 Webster]

WordNet (r) 3.0 (2006):

diarthrosis
    n 1: a joint so articulated as to move freely [syn: synovial
         joint, articulatio synovialis, diarthrosis]