Part of the Distillation process of an Essential Oil. After the Distillation process is complete, the distilled water at the bottom of the kettle is the Hydrosol. It contains tiny molecules of the pure essential oil in the distilled water. Safe to use directly on the skin because of the dilution percentage. Several can be used for Toners---we have those available for use with our Facial Care packs. Others can be used for a Body Splash or Room Freshener.