Final answer:
Mediastinal shift is the collapse of the opposite side of the thorax due to uncontrolled increased positive pressure in one side.
Step-by-step explanation:
The collapse of the opposite side of the thorax due to uncontrolled increased positive pressure in one side is called mediastinal shift. Medistinal shift occurs when the pressure in one side of the thorax pushes the mediastinum (the area between the lungs) to shift to the opposite side, causing lung compression and collapse.