soft water

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

• noun Water containing little or no dissolved salts of calcium or magnesium, especially water containing less than about 85 parts per million of calcium carbonate.

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

• noun chemistry Water with a low concentration of dissolved minerals, especially calcium, making it easier to lather with soap.

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

• noun water that is not hard (does not contain mineral salts that interfere with the formation of lather with soap)