reverse osmosis

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A process by which a solvent such as water is purified of solutes by being forced through a semipermeable membrane through which the solvent, but not the solutes, can pass.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun chemistry a technique whereby a solution is forced through a semipermeable membrane under pressure; used to generate drinkable water from sea water, or to separate chemical compounds

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (chemistry) a method of producing pure water; a solvent passes through a semipermeable membrane in a direction opposite to that for natural osmosis when it is subjected to a hydrostatic pressure greater than the osmotic pressure