solar plexus, dense cluster of nerve cells and supporting tissue, located behind the stomach in the region of the celiac artery just below the diaphragm. It is also known as the celiac plexus. Rich in ganglia and interconnected neurons, the solar plexus is the largest autonomic nerve center in the abdominal cavity (see nervous system). Through branches it controls many vital functions such as adrenal secretion and intestinal contraction. Popularly, the term "solar plexus" may refer to the pit of the stomach. A blow to that area, if it penetrates to the true solar plexus, not only causes great pain but may also temporarily halt visceral functioning.