pneumomediastinum

noun

pneu·​mo·​me·​di·​as·​ti·​num ˌn(y)ü-mō-ˌmēd-ē-ə-ˈstī-nəm 

plural pneumomediastina -nə 

1

: an abnormal state characterized by the presence of gas (as air) in the mediastinum

2

: the induction of pneumomediastinum as an aid to radiography