Crohn's disease, is an inflammatory condition that can be found in any part of the gastrointestinal tract, from the mouth to the anus, but most commonly it involves the lower end of the small intestine (ileum) and/or the large intestine (colon). Areas of inflammation can appear interspersed with healthy parts of the intestine and may involve the full thickness (all layers) of the intestine.

Sometimes different names are used to describe the part of the intestine that is involved. When Crohn's is confined to the ileum it may be referred to as 'ileitis', whereas for the ileum and colon it is 'ileocolitis', and when only the colon is involved it is called 'Crohn's colitis', or shortened to just 'colitis', which can be confusing. Most people prefer to just call it 'Crohn's'.