Medical Definition of IBD (inflammatory bowel disease) IBD (inflammatory bowel disease): A group of chronic intestinal diseases characterized by inflammation of the bowel -- the large or small intestine. The most common types of inflammatory bowel disease are ulcerative colitis and Crohn's disease. Crohn's disease favors the ileum (the lower part of the small intestine) but can occur anywhere along the intestinal tract while, by contrast, ulcerative colitis affects the colon (the large intestine) alone.