Find the word definition

Wiktionary
pneumomediastinum

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

Wikipedia
Pneumomediastinum

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.