Peritonitis is defined as an inflammation of the peritoneum, the thin membrane that lines the abdominal wall and covers most of the organs of the body. It is often caused by the introduction of an infection into the otherwise sterile peritoneal environment as a result of perforated bowel, such as a ruptured appendix or colonic diverticulum. The disease may also be caused by introduction of a chemically irritating material, such as gastric acid from a perforated ulcer or bile from a perforated gall bladder or a lacerated liver.

Peritonitis is often clinically seen as a constellation of signs and symptoms, which includes abdominal pain, tenderness on palpation, abdominal wall muscle rigidity, and systemic signs of inflammation of an infection. It is often caused by the introduction of an infectious agent into the otherwise sterile peritoneal environment. The peritoneum reacts to a variety of pathologic stimuli, with a fairly uniform inflammatory response. Depending on the underlying pathology, the resultant peritonitis may be infectious or sterile (i.e., chemical or mechanical).