Source : Webster's Revised Unabridged Dictionary (1913)

Pneumothorax \Pneu`mo*tho"rax\, n. [Gr. ? air + E. thorax.]
   (Med.)
   A condition in which air or other gas is present in the
   cavity of the chest; -- called also {pneumatothorax}.

Source : WordNet®

pneumothorax
     n : abnormal presence of air in the pleural cavity resulting in
         the collapse of the lung; may be spontaneous (due to
         injury to the chest) or induced (as a treatment for
         tuberculosis)