This is a reduced of a section of text in the book Karate technique selection & Street Fighting Statistics & Medical Outcomes...

The Solar Plexus (a common target in karate and the martial arts) is more often called the celiac plexus in medical terminology. Striking the celiac plexus can cause the diaphragm to spasm, resulting in difficulty in breathing — a sensation commonly known as "getting the wind knocked out of you".

A dense cluster of nerve cells and supporting tissue, are located behind the stomach in the region of the celiac artery just below the diaphragm. It is also known as the celiac plexus and one of its functions is breathing control.

A blow to this region can also affect the celiac plexus itself, possibly interfering with the functioning of the viscera, as well as causing great pain perceived to come from the viscera (as the viscera nerves run through this plexus). It carries pain “messages” from the pancreas, liver, kidney, gall bladder, spleen and bowels. It may also temporarily halt visceral functioning.