Peritonitis is an inflammation

Inflammation

Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...

 of the peritoneum

Peritoneum

The peritoneum is the serous membrane that forms the lining of the abdominal cavity or the coelom — it covers most of the intra-abdominal organs — in amniotes and some invertebrates...

, the serous membrane

Serous membrane

In anatomy, serous membrane is a smooth membrane consisting of a thin layer of cells which secrete serous fluid. Serous membranes line and enclose several body cavities, known as serous cavities, where they secrete a lubricating fluid which reduces friction from muscle movement...

 that lines part of the abdominal cavity

Abdomen

In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...

 and viscera. Peritonitis may be localised or generalised, and may result from infection

Infection

An infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...

 (often due to rupture of a hollow organ

Gastrointestinal perforation

Gastrointestinal perforation is a complete penetration of the wall of the stomach, small intestine or large bowel, resulting in intestinal contents flowing into the abdominal cavity. Perforation of the intestines results in the potential for bacterial contamination of the abdominal cavity...

 as may occur in abdominal trauma

Abdominal trauma

Abdominal trauma is an injury to the abdomen. It may be blunt or penetrating and may involve damage to the abdominal organs. Signs and symptoms include abdominal pain, tenderness, rigidity, and bruising of the external abdomen. Abdominal trauma presents a risk of severe blood loss and infection...

 or appendicitis

Appendicitis

Appendicitis is a condition characterized by inflammation of the appendix. It is classified as a medical emergency and many cases require removal of the inflamed appendix, either by laparotomy or laparoscopy. Untreated, mortality is high, mainly because of the risk of rupture leading to...

) or from a non-infectious process.
The main manifestations of peritonitis are acute abdominal pain

Abdominal pain

Abdominal pain can be one of the symptoms associated with transient disorders or serious disease. Making a definitive diagnosis of the cause of abdominal pain can be difficult, because many diseases can result in this symptom. Abdominal pain is a common problem...

, abdominal tenderness, and abdominal guarding

Abdominal guarding

Abdominal guarding is the tensing of the abdominal wall muscles to guard inflamed organs within the abdomen from the pain of pressure upon them. The tensing is detected when the abdominal wall is pressed...

, which are exacerbated by moving the peritoneum, e.g., coughing (forced cough may be used as a test), flexing one's hips, or eliciting the Blumberg sign

Blumberg sign

Blumberg's sign is a sign that is elicited during physical examination in medicine. It is indicative of peritonitis.The abdominal wall is compressed slowly and then rapidly released. Presence of pain makes the sign positive...

 (a.k.a.