Mucositis: Inflammation of the mucous membranes lining the digestive tract from the mouth to the anus. Mucositis is a common side effect of chemotherapy and of radiotherapy that involves any part of the digestive tract. Mucositis affects the rapidly dividing mucosal cells that line the mouth, throat, stomach, and intestines, which normally have a short lifespan. If a therapy destroys these cells, they may not be replaced right away, in which case mucositis results. A person with mucositis may have raw sores (ulcers) in the mouth and throat and feel like he or she has a sunburn in the throat.