The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pneumothorax
    n 1: 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)