1. An effusion of watery liquid into the cavity of a joint: Dr. McMahon explained that typically there is an amount of hydrarthrosis in the articulating connections between the bones consisting of fluid which lubricates them.
2. Inflammation and swelling of a movable joint because of excess synovial fluid: When a person is suffering with hydrarthrosis, it is a result of a painful condition such as "water on the knee" which may require surgery to drain.