hydrothorax n 1: 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
Hydrothorax \Hy`dro*tho"rax\, n. [Hydro-, 1 + thorax.] (Med.) An accumulation of serous fluid in the cavity of the chest. [1913 Webster]