“Indigestion” refers to any number of gastrointestinal complaints, which can include gas (belching or flatulence) and upset stomach. “Heartburn” refers to a burning feeling that can be caused by stomach acid regurgitating into the esophagus from the stomach, by gastritis (inflammation of the lining of the stomach), or by an ulcer of the stomach or duodenum (also called peptic ulcer). In some cases, such as lactose intolerance, symptoms of indigestion are due to a specific cause that requires specific treatment. Sometimes, symptoms associated with indigestion are caused by diseases unrelated to the gastrointestinal tract. The following conditions are related to the digestive system.