Appendicitis: Inflammation of the appendix, usually associated with infection of the appendix. Appendicitis often causes fever, loss of appetite, and pain. Appendicitis may be suspected because of the medical history and physical examination. The pain of appendicitis can be located in various areas of the belly. If the appendix ruptures and infection spreads throughout the abdomen, the pain becomes widespread as the entire lining of the abdomen becomes inflamed. Ultrasonography and computerized tomography may be helpful in diagnosis.