Bed sore: A painful, often reddened area of degenerating, ulcerated skin caused by pressure and lack of movement, and worsened by exposure to urine or other irritating substances on the skin. Untreated bed sores can become seriously infected or gangrenous. Bed sores are a major problem for patients who are confined to bed or a wheelchair. They can be prevented by moving the patient frequently, changing bedding, and keeping the skin clean and dry. Also called a pressure sore, decubitus sore, or decubitus ulcer.