Source : Webster's Revised Unabridged Dictionary (1913)

Hydrothorax \Hy`dro*tho"rax\, n. [Hydro-, 1 + thorax.] (Med.)
   An accumulation of serous fluid in the cavity of the chest.

Source : WordNet®

hydrothorax
     n : accumulation of fluid in the pleural cavity (the space
         between the lungs and the walls of the chest) often
         resulting from disease of the heart or kidneys