Can be congenital (as in Hirschsprung's disease) or acquired (as when children refuse to defecate)

• An abnormal enlargement of the colon

Wiktionary Word definitions in Wiktionary
n. 1 (context uncountable English) abnormal dilatation of the colon, not caused by mechanical obstruction. 2 (context countable English) A colon thus enlarged.

WordNet Word definitions in WordNet
n. an abnormal enlargement of the colon; can be congenital (as in Hirschsprung's disease) or acquired (as when children refuse to defecate)

Wikipedia Word definitions in Wikipedia
Megacolon is an abnormal dilation of the colon (also called the large intestine ). The dilation is often accompanied by a paralysis of the peristaltic movements of the bowel. In more extreme cases, the feces consolidate into hard masses inside the colon, ...