Source : Webster's Revised Unabridged Dictionary (1913)

Hydronephrosis \Hy`dro*ne*phro"sis\, n. [NL., Gr. "y`dwr water +
   ? a kidney.] (Med.)
   An accumulation of urine in the pelvis of the kidney,
   occasioned by obstruction in the urinary passages.

Source : WordNet®

hydronephrosis
     n : accumulation of urine in the kidney because of an
         obstruction in the ureter