Diarrhoea is the frequent and excessive discharge of watery fluid from the bowel. Diarrhoea is really a symptom of disease rather than a disease itself.
Absolutely everyone experiences diarrhoea at some time.
The ten metres of an adult human gut is very sensitive to irritants, and any irritation of the lower half, particularly the last two metres (the large intestine), will result in diarrhoea.
Diarrhoea may be considered by a patient to be the more frequent passing of motions that are softer than usual, but to be medically significant, the motions must be at least part liquid and be passed more than four times a day.