A collection of air in the pleural cavity, into which it has gained entrance by a defect in the lung or a wound in the chest wall. When air enters the chest, the lung immediately collapses towards the centre of the chest; however, air being absorbed from the pleural cavity, the lung expands again within a short time. (See LUNGS, DISEASES OF.)

Tension pneumothorax is a life-threatening condition in which the air in the hemithorax is under such pressure that it forces the heart to the other side and compresses the still-inflated lung on the other side. It must be promptly relieved by inserting a hollow tube into the pleural cavity – a chest drain to allow the trapped air to escape.