For a long time people have been told that caffeine is a diuretic. For some, this translates into advice to avoid or remove caffeinated beverages from the diet of people at risk of dehydration, or during periods of extreme summer heat. 
While possibly well meaning, this advice is wrong.
By definition, a diuretic is a product that increases the body’s production of urine. Hence water, or any drink consumed in large volumes, is a diuretic. Importantly, urinating more does not inevitably lead to dehydration (excessive loss of body water).
Drinking simultaneously provides the body with fluid for absorption (avoiding dehydration) and initiates urine production. Depending on the urine losses that occur following drinking, a beverage might be more accurately described as a “poor _re_hydrator” if large fluid losses result.
