Nano is a SI prefix denoting 10-9. Nanofiltration is a term to describe membranes which will remove particles greater in size than 10-9 meters. The 'tightest' Nanofiltration membranes cross over with the 'slackest' of reverse osmosis membranes.

At this degree of filtration, some dissolved ions are removed - generally the larger and more highly charged ions such Ca2+, Mg2+ and SO42-. Some membranes are specially formulated to remove particular ions and have specific designations - eg Koch Membrane Systems Model 8923S-400 which is designated as a Softening membrane due to its ability to exclude Ca2+ ions whilst allowing Na1+ ions to pass.

Another application of nanofiltration membranes is colour removal from surface water due to the ability to exclude the shortest chain organic molecules which are able to pass though the 'slacker' MF and UF membranes.