The appendix is a small, tube-shaped organ attached to your large intestine. It sits in the lower right side of your abdomen, about where your hip meets your belly. For a long time, scientists thought the appendix served no purpose, a leftover from our evolutionary past. More recent research suggests it might help store helpful bacteria that keep your digestive system healthy, though you can live perfectly well without it.

The appendix becomes important when it gets infected and swollen, a painful condition called appendicitis. When this happens, doctors usually remove it surgically before it ruptures. Appendicitis causes sharp pain in the lower right abdomen, and it's one of the most common reasons for emergency surgery in children and adults.

The word also means a section at the end of a book or document that provides extra information. A history textbook might have an appendix with maps, a science book might include an appendix with extra data tables, and a novel might have an appendix explaining historical context. Like the organ, a book's appendix is attached to the main body but contains supplemental material that supports the main content without being essential to it.