Deionized water contains virtually no dissolved minerals. It's essential for laboratory work, spot-free rinse, precision manufacturing, hydroponics, and medical applications.
Deionized (DI) water has had virtually all dissolved mineral ions — calcium, magnesium, sodium, iron, copper, chloride, sulfate — removed through ion exchange resin. What remains is pure H₂O with a TDS near zero.
DI water is required anywhere minerals in water would interfere with a process or leave deposits. Car detailers use it for spot-free rinse. Lab technicians use it for reagent preparation. Hydroponic growers use it as a clean base for nutrient solutions. Electronics manufacturers use it for circuit board cleaning. A standard carbon filter cannot produce DI water — it requires ion exchange resin.