1. a large blister, containing serous fluid.
2. (in anatomy) a rounded bony prominence.
3. a thin-walled air-filled space within the lung, arising congenitally or in emphysema. It may cause trouble by rupturing into the pleural space (see pneumothorax), by adding to the air that does not contribute to gas exchange, and/or by compressing the surrounding lung and making it inefficient. --bullous adj.