Milli-Q water

Milli-Q refers to water that has been purified and deionized to a high degree (typically 18.2 MΩ·cm) by a water purification systems manufactured by Millipore Corporation. It is also a registered trademark of Millipore. The term is also used to refer to water produced by any similar system.
Milli-Q water uses resin filters and deionization to purify the water. The system monitors the ion concentration by measuring the electrical resistivity of the water. Higher resistivity means fewer charge-carrying ions. Most Milli-Q systems dispense the water through a 0.22 μm membrane filter.
It is capable of producing water pure enough to get reproducibility and accuracies within parts per million when using ion mass spectrometers in analytical geochemistry.^[1] Other users of this system include microbiology^[2] and atmospheric science.^{[3]
Source: Wikipedia}