A pneumothorax is a condition in which air has entered and expanded the normally closed pleural space, driving pleural pressure up toward atmospheric pressure, and resulting in partial or complete collapse of the lung. When pleural pressure approaches zero, the lung and chest wall both move toward the equilibrium positions they would assume in the absence of any external pressures-- the lung collapses and the chest wall springs out.
A pneumothorax may be induced by a break in either the parietal pleura (e.g., from trauma, needle or catheter insertion) or in the visceral pleura (e.g., from rupture of a subpleural air pocket or necrosis of lung adjacent to the pleura).
The right side of this patient's thoracic cavity (viewer's left) shows a darker area where the lung should be. Note the expanded chest wall and the collapsed lung.