U.S. National Library of Medicine.
Pneumatosis is the abnormal presence of air or other gas within tissues.In the lungs, emphysema involves enlargement of the distal airspaces, and is a major feature of chronic obstructive pulmonary disease (COPD). Other pneumatoses in the lungs are focal (localized) blebs and bullae, pulmonary cysts and cavities. Pneumoperitoneum (or peritoneal emphysema) is air or gas in the abdominal cavity, and is most commonly caused by gastrointestinal perforation, often the result of surgery. Pneumarthrosis, the presence of air in a joint, is rarely a serious sign. Lung cysts
A lung cyst, or pulmonary cyst, encloses a small volume of air,  and has a wall thickness of up to 4 mm. A minimum wall thickness of 1 mm has been suggested, but thin-walled pockets may be included in the definition as well. Pulmonary cysts are not associated with either smoking or emphysema.A lung cavity has a wall thickness of more than 4 mm. Other thoracic
Pneumothorax, air or gas in the pleural space
Pneumomediastinum, air or gas in the mediastinum
Also called mediastinal emphysema or pneumatosis/emphysema of the mediastinum

Abdominal
Pneumoperitoneum (or peritoneal emphysema), air or gas in the abdominal cavity. The most common cause is a perforated abdominal viscus, generally a perforated peptic ulcer, although any part of the bowel may perforate from a benign ulcer, tumor or abdominal trauma. Pneumatosis intestinalis, air or gas cysts in the bowel wall
Gastric pneumatosis (or gastric emphysema) is air or gas cysts in the stomach wall

Joints
Pneumarthrosis is the presence of air in a joint.