Toxic megacolon is  acute dilatation of the non-obstructed colon, total or segmental,
accompanied by signs of systemic toxicity, which occurs as a consequence of severe inflammation of the colon.

Classically, toxic megacolon was considered to be exclusively related to severe ulcerative colitis but it is now known that it may also complicate severe forms of Crohn's disease or pseudomembranous colitis.