What is "pneumomediastinum"

n. The condition of having the presence of air in the mediastinum.

Pneumomediastinum (from Greek pneuma - "air", also known as mediastinal emphysema) is pneumatosis (abnormal presence of air or other gas) in the mediastinum. First described in 1819 by René Laennec, the condition can result from physical trauma or other situations that lead to air escaping from the lungs, airways, or bowel into the chest cavity.