Demineralized water, often called deionized water, is water from which the dissolved mineral ions have been removed by passing it through ion exchange resins. The result is chemically similar to distilled water but produced by an entirely different route, and the difference in method explains where each one is used and where each one falls short.
How the resins do it
Two beds do the work. A cation resin exchanges positively charged ions, calcium, magnesium and sodium among them, for hydrogen. An anion resin exchanges negatively charged ions, chloride, sulfate and bicarbonate, for hydroxide. The hydrogen and hydroxide released combine into water, so what leaves the beds has had its dissolved salts replaced by more water. Mixed bed units combine both resins in one vessel for a higher purity finish. The resins are consumed and are either regenerated with acid and caustic or replaced.
How it differs from distilled water
Distillation changes phase, so it leaves behind everything that does not evaporate: minerals, salts and also bacteria, viruses and most organic matter. Demineralization removes only what carries a charge. That means dissolved ions go, but non ionic organics, particulates and micro organisms pass through untouched unless other stages handle them. So demineralized water can have very low conductivity and still be microbiologically unsuitable, which is why it is not treated as a drinking water product in the way distilled water is.
Where it is actually used
Laboratory work, where ionic contamination would interfere with results. Battery topping up, steam raising and cooling systems, where minerals would scale. Cosmetic and pharmaceutical manufacture. Domestically the most visible use is spot free rinsing: car detailing and window cleaning, where a deionizing cartridge on a hose gives a final rinse that dries without leaving any mark, because there is nothing dissolved left to deposit. The USGS classes water as soft at 0 to 60 mg/L as calcium carbonate, and demineralized water sits far below that.
The running cost, honestly
This is what surprises people. Resin capacity is consumed in proportion to how much was dissolved in the water going in, so on hard water a deionizing cartridge exhausts quickly and the cost per volume is high. Softening the feed first extends resin life considerably and is standard practice where the volume is significant. A conductivity or TDS meter tells you when the resin is spent, since exhaustion gives no visible sign. For drinking water, reverse osmosis achieves a comparable result at a fraction of the running cost.
Questions people ask about what is demineralized water
Is demineralized water the same as distilled?
Chemically similar for dissolved ions, but produced differently. Demineralization removes charged ions only, leaving organics and micro organisms.
Is it safe to drink?
It is not sold or treated as a drinking water product, because ion exchange does not remove bacteria, viruses or non ionic organics.
What is it used for at home?
Mostly spot free rinsing for cars and windows, where nothing dissolved remains to leave a mark as the water dries.
Why is it expensive to produce?
Resin capacity is consumed in proportion to what was dissolved in the feed, so hard water exhausts a cartridge quickly.