Final answer:
The term for a sunken decorative relief panel is "sunken relief", an art technique used extensively in Ancient Egyptian and Islamic art that features designs carved below the surface level.
Step-by-step explanation:
The term for a sunken decorative relief panel is sunken relief. This artistic technique involves carving the image below the level of the surrounding surface so that the details of the design are lowered rather than raised. Sunken reliefs are especially prominent in Ancient Egyptian art, where they were well suited to the bright sunlight of the region. Notable examples can be seen in the decorations of tombs like that of Ramses IV, which include hieroglyphs executed in this style. The practice was not limited to Egypt, however, and can also be found in Islamic art, such as the decorative panels from Samarra, Iraq.
Sunken relief offers a distinctive visual effect as the design does not cast deep shadows, which makes it highly legible under direct lighting conditions. The technique also allows for delicate carvings that might be too fragile if done in high relief. Sunken reliefs have been documented to be used decoratively on items such as tomb walls, ivories, and even in architecture, serving both functional and aesthetic purposes.