What area is defined as the septum or cavity between two principal portions of an organ, specifically separating the lungs?

The mediastinum is the area defined as the septum or cavity between the two principal portions of an organ, specifically separating the lungs. It is a central compartment located in the thoracic cavity and is crucial because it contains important structures such as the heart, trachea, esophagus, and great blood vessels. The mediastinum serves a vital role in dividing the thoracic cavity into left and right halves, thereby providing structural support and allowing for the proper functioning of the lungs and heart.

The other choices pertain to different anatomical structures or areas. The pleura refers to the membranes surrounding the lungs, providing lubrication and protection. The thorax is a broader term that encompasses the entire chest region, including the ribs and intercostal muscles. The pericardium is the surrounding sac of fibrous tissue that encloses the heart. Each of these has its specific roles, but none serves the purpose of separating the lungs in the manner described in the question.