It is not the same thing. Deionized water may contain neutral organic molecules, including viruses and bacteria, as it uses ion exchange resins pre-loaded with protons and hydroxides to remove all other anions and cations. Distilled water has been vaporized out of one container and recondensed into another, so contains nothing but water and molecules that form azeotropes with water.

Distilled water is deionized, but deionized has not necessarily been distilled.

In practice, however, both are overkill for baby formula.

Nobody's arguing that they're identical processes. My point is about the _labeling_ differences. Distilled water is often sold as deionized water in various parts of the world.

But there is literally zero additional printing cost for using the word that precisely matches the purification process. They even have the same number of letters in English. Why would anybody do that?