the presence of air or gas in a pleural cavity, esp. as a result of perforation or rupture of the lung tissue
See pneumothorax in American Heritage Dictionary 4
(no͞oˌmō-thôrˈăksˌ, -thōrˈ-, nyo͞oˌ-)
noun
Accumulation of air or gas in the pleural cavity, occurring as a result of disease or injury, or sometimes induced to collapse the lung in the treatment of tuberculosis and other lung diseases.