The Collaborative International Dictionary of English v.0.48:

Mediastine \Me`di*as"tine\, Mediastinum \Me`di*as*ti"num\, n.
   [NL. mediastinum, fr. L. medius middle; cf. mediastinus
   helper, a menial servant, LL. mediastinus equiv. to medius:
   cf F. m['e]diastin.] (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]

WordNet (r) 3.0 (2006):

mediastinum
    n 1: the part of the thoracic cavity between the lungs that
         contains the heart and aorta and esophagus and trachea and
         thymus