Atelectasis (at-uh-LEK-tuh-sis) is the collapse of a lung or part of a lung, also known as a lobe. It happens when tiny air sacs within the lung, called alveoli, lose air.

Atelectasis is one of the most common breathing complications after surgery. It's also a possible complication of other respiratory problems, including cystic fibrosis, lung tumors, chest injuries, fluid in the lung and respiratory weakness. You may develop atelectasis if you breathe in a foreign object.

This condition can make breathing hard, particularly if you already have lung disease. Treatment depends on what's causing the collapse and how severe it is.

The definition of atelectasis is broader than pneumothorax (noo-moe-THOR-aks). Pneumothorax is when air leaks into the space between your lungs and chest wall, causing part or all of a lung to collapse. Pneumothorax is one of several causes of atelectasis.