The Collaborative International Dictionary of English v.0.48:

Synarthrosis \Syn`ar*thro"sis\, n.; pl. Synarthroses. [NL.,
   fr. Gr. ? a being jointed together, fr. ? to link or joint
   together; sy`n with + ? a joint.] (Anat.)
   Immovable articulation by close union, as in sutures. It
   sometimes includes symphysial articulations also. See the
   Note under Articulation, n., 1.
   [1913 Webster]