To me, chronic diarrhea is a diarrhea that lasts more than two weeks. There are plenty of illnesses that will give you a brief period of diarrhea, and as long as the child remains well-hydrated and there is no blood in the stool, I don't get very worried. There are infections, though, that can cause a prolonged diarrhea. More than two weeks is when I start to send stool studies out to look for interesting things and stuff like that. There are other causes of chronic diarrhea, like inflammatory bowel disease and Ulcerative Colitis and Crohn's disease, but those tend to present with more cramping. In addition to the high output, but also with blood in the stool. Those would be obvious to you, if that is happening. What I suggest if you are having diarrhea, if hydration becomes an issue or diarrhea that lasts for more than two weeks, see the pediatrician. Have your pediatrician send out for those stool studies.