Source : Webster's Revised Unabridged Dictionary (1913)

Hydrarthrosis \Hy`drar*thro"sis\, n. [NL. See {Hydro-}, 1, and
   {Arthrosis}.] (Med.)
   An effusion of watery liquid into the cavity of a joint.

Source : WordNet®

hydrarthrosis
     n : inflammation and swelling of a movable joint because of
         excess synovial fluid