Megacolon

n. (Medicine) colon that is large or dilated in an abnormal manner

Megacolon is an abnormal dilatation of the colon (a part of the large intestines) that is not caused by mechanical obstruction. The dilatation is often accompanied by a paralysis of the peristaltic movements of the bowel, resulting in chronic constipation. In more extreme cases, the feces consolidate into hard masses inside the colon, called fecalomas (literally, fecal tumor), which require surgery to be removed.
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Megacolon /mega·co·lon/ dilatation and hypertrophy of the colon. acquired megacolon colonic enlargement associated with chronic constipation, but [more]Megacolon - Community and Resources

Noun
1. an abnormal enlargement of the colon; can be congenital (as in Hirschsprung's disease) or acquired (as when children refuse to defecate)
(hypernym) colon

An abnormally enlarged colon.