The term for the accumulation of air in the chest cavity causing lung collapse is pneumothorax.
More precisely, the air collects in the pleural space, the narrow gap between the lung and chest wall. Normally, this space maintains negative pressure that helps keep the lung expanded. When air enters it—through an injury, a ruptured air pocket, or certain lung diseases—the pressure balance is disrupted and part or all of the lung can collapse.
A pneumothorax may happen spontaneously or after trauma or a medical procedure. A tension pneumothorax is a dangerous form in which pressure keeps building, potentially compressing the heart and the other lung. Typical symptoms include sudden, sharp one-sided chest pain and shortness of breath.
The word combines Greek pneuma, meaning “air” or “breath,” with thorax, meaning “chest.” A hemothorax contains blood, a hydrothorax contains watery fluid, and a chylothorax contains lymphatic fluid—similar conditions, but not the same diagnosis.