{ ‖ } n. [ NL. mediastinum, fr. L. medius middle; cf. mediastinus helper, a menial servant, LL. mediastinus equiv. to medius: cf F. médiastin. ] (Anat.) 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. [ 1913 Webster ]