specific gravity

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

• noun The ratio of the mass of a solid or liquid to the mass of an equal volume of distilled water at 4°C (39°F) or of a gas to an equal volume of air or hydrogen under prescribed conditions of temperature and pressure.

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

• noun physics The ratio of the mass of a substance to that of an equal volume of water at 4°C (or to some other reference substance)

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

• noun the density of a substance relative to the density of water