It's De-Ionized Water. Simply put, it is a process that the water goes through to remove the "Total Dissolved Solids" leaving you with a spot free finish without having to dry the vehicle. However, the tanks are not cheap and need to be recharged after so many gallons have passed throught them. Sure is nice though to just wash the vehicle and move immediately to the interiors...saves me about 40 min. a day in dry time.