A condition in which there is a tendency to overactivity of the bowel emptying activity, accompanied by abdominal pain and diarrhea; the cause of the condition is unknown but psychological and allergic factors are generally believed to play a role.
Biology Dictionary
Definition:
Irritable bowel syndrome is a chronic functional disorder of abdominal pain, altered bowel habits and stool consistency which is not due to a discernable physical or biochemical reason. There is often a psychological component to its cause.
WordNet Dictionary