At its most basic level, alkaline water is water that has a pH of higher than 7. The pH simply refers to the measurement of relative hydrogen ions. The fewer hydrogen ions, the higher the pH, and the higher the pH, the higher the alkalinity.
Natural alkaline water (not to be confused with artificial alkaline water) is water that has a pH of over 7 due to its naturally high mineral content.
Natural alkaline water can be found in a handful of remote locations around the world. Water, for instance, that flows through porous volcanic rock and is untouched by human pollutants, is infused with alkaline minerals and thus has a naturally high pH. Indeed, it is the water from these locales (and its healthful effects on humans) that inspired the scientific inquiry into the properties and benefits of alkaline water.