Capacitive deionization (CDI) is really a technology to deionize water by using an electrical potential difference over two porous carbon electrodes. Anions, ions which has a negative charge, are taken from the water and so are stored inside positively polarized electrode. Likewise, cations (positive charge) are held in the cathode, and that is the negatively polarized electrode.
Today, CDI is usually used to the desalination of brackish water, and that is water that has a low or moderate salt concentration (below 10 g/L). Other technologies for your deionization of water are, and the like, distillation, reverse osmosis and electrodialysis. Compared to reverse osmosis and distillation, CDI is recognized as an energy-efficient technology for brackish water desalination. This is principally because CDI removes the salt ions from the stream, whilst the other technologies extract water from the salt solution.
Historically, CDI has become referred to as electrochemical demineralization, "electrosorb process for desalination of water", or electrosorption of salt ions. It also passes by the names of capacitive desalination, or perhaps the commercial literature as "CapDI".