Pure De-ionised water.  Used to prevent furring and scale formation. For use commonly in steam irons and re-fillable batteries.  Also used in window cleaning and due to containing no minerals it will not leave streaks or deposits when dry. Additionally polished and then filtered with a UV lamp the micro-siemen count is typically 5 and always less than 10.  (or approximately 10ppm).  Protect from daylight for storage.  Distilled water has the same properties  but it is derived by steam distillation.