The mediastinum is the central part of the thoracic (chest) cavity. It is not a distinct organ, but a volume of space that has been arbitrarily defined to make it easier to discuss the location of structures in the chest.

The mediastinum is bounded by the front by the sternum or breastbone, to the back by the spine, and to the left and right by the pleurae of the lungs. The heart, the thymus, and a number of lymph nodes are wholly contained inside the mediastinum; passing through it are a number of major blood vessels, lymph vessels, and nerves, along with portions of the esophagus, trachea, and bronchi.

The mediastinum is divided for convenience into several sub-volumes that are named using the conventional medical terminology for top (superior), bottom (inferior), front (anterior), and back (posterior). The superior mediastinum is that part of the chest cavity which is above the highest point of the heart. The great vessels of the heart (aorta, superior vena cava, pulmonary vessels) extend up from the heart into the superior mediastinum. The superior mediastinum also contains the thymus, portions of the esophagus and trachea, and other structures. The inferior mediastinum includes the whole chest cavity below the superior mediastinum, and is subdivided into the middle mediastinum (the heart and pericardial sac), the anterior mediastinum (the small space between heart and sternum, mostly filled with fatty tissue), and the posterior mediastinum (the space between the heart and the spine, containing much of the esophagus and a variety of arteries, veins, and nerves).