Definition of superior mediastinum in US English:

superior mediastinum

noun

Anatomy
  • The upper part of the mediastinum; the part of the mediastinum above the heart; specifically the part of the mediastinum extending downwards from the thoracic inlet to a plane passing between the manubrium of the sternum and the fourth thoracic vertebra.

Origin

Early 19th century. After post-classical Latin mediastinum superius.