A Pneumothorax or a collapse of a lung refers to a condition where a space between the lung and the thoracic cavity is filled with air, which causes the lung to collapse. The resultant collapse can be a partial or complete depending upon the degree of affection.
The lung is surrounded by thin double layered sheath viz. pleurae. The pleura consists of two layers; the inner layer i.e. visceral pleura which holds the lungs and the outer layer i.e. parietal pleura which separates the lung from the chest wall and other organs.
Air can enters into the space between two pleurae, through a trauma or an injury to the chest wall. The space is known as a pleural space. This can result in a Pneumothorax.