Peritonitis is an inflammation of the peritoneum, the thin membrane lining the abdominal cavity and covering abdominal organs. It can result from various causes, including bacterial or fungal infections, perforation of abdominal organs, or complications from conditions like appendicitis or pancreatitis. Symptoms typically include severe abdominal pain, fever, and nausea, and it is considered a medical emergency requiring prompt diagnosis and treatment, often involving antibiotics and surgery.