wetting agent

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

• noun A substance that reduces the surface tension of water or another liquid, causing the liquid to spread across or penetrate more easily the surface of a solid.

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

• noun a substance, such as a surfactant, that reduces the surface tension of a liquid, allowing it to spread across, and penetrate the surface of a solid

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

• noun a chemical agent capable of reducing the surface tension of a liquid in which it is dissolved