The passage of loose or watery stools (greater than 8 oz or 200 g per 24 h), usually at more frequent than normal intervals. The two principal mechanisms responsible for diarrhea are the rise in osmotic pressure of fecal contents, diminishing the absorption of water by the intestine (osmotic diarrhea); and the increase in electrolyte secretion into the intestinal lumen by its epithelial cells, leading to an increased flow of water from the intestinal mucosa into the lumen (secretory diarrhea).