Plastic regions refer to areas within a material where deformation occurs without returning to the original shape after the applied stress is removed. This behavior is common in materials like metals and polymers, where they can undergo permanent changes in structure when subjected to sufficient force. In the context of geology, plastic regions can also describe layers of the Earth's crust that behave plastically under pressure, allowing for the movement of tectonic plates. These regions are crucial in understanding phenomena such as earthquakes and volcanic activity, as they can store and release energy over time.