Mediastinum

A partition; a septum; specifically, the folds of the pleura (and the space included between them) which divide the thorax into a right and left cavity. The space included between these folds of the pleura, called the mediastinal space, contains the heart and gives passage to the esophagus and great blood vessels.

the part of the thoracic cavity between the lungs that contains the heart and aorta and esophagus and trachea and thymus