How is diarrhea defined?

Diarrhea can be defined in absolute or relative terms based on either the frequency of bowel movements or the consistency (looseness) of stools.

Frequency of bowel movements. Absolute diarrhea is having more bowel movements than normal. Thus, since among healthy individuals the maximum number of daily bowel movements is approximately three, diarrhea can be defined as any number of stools greater than three.
Relative diarrhea is having more bowel movements than usual.

Thus, if an individual who usually has one bowel movement each day begins to have two bowel movements each day, then diarrhea is present-even though there are not more than three bowel movements a day, that is, there is not absolute diarrhea.

Consistency of stools. Absolute diarrhea is more difficult to define on the basis of the consistency of stool because the consistency of stool can vary considerably in healthy individuals depending on their diets. Thus, individuals who eat large amounts of vegetables will have looser stools than individuals who eat few vegetables.

Stools that are liquid or watery are always abnormal and considered diarrheal. Relative diarrhea is easier to define based on the consistency of stool. Thus, an individual who develops looser stools than usual has diarrhea--even though the stools may be within the range of normal with respect to consistency.