Fluid in the abdomen that is outside of the digestive tract (outside of the organs) is called "ascites." Ascites can appear for many reasons, but none of them are normal.
The most common causes of ascites are things that interfere with normal blood circulation. The body's circulation is a bit of an obstacle course. Some blood needs to travel through the liver on its way back to the heart. This doesn't work well if the liver is scarred (a problem called cirrhosis).
In this case, fluid from blood that passes through the liver can seep from the liver into the abdomen. Heart failure is another common condition that interferes with circulation and results in fluid in the abdomen.
Inflammation or infection can cause a small amount of fluid to collect around the area that is involved. For example, it is common to have a small amount of fluid around an infected gallbladder.
The smooth tissue that lines the front wall of the abdomen is named the "peritoneum." Cancer or infection that has spread to the peritoneum causes fluid to accumulate in the abdomen. An example of a cancer that can spread to the peritoneum is ovarian cancer.