equivalent weight

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

• noun The weight of a substance that will combine with or replace one mole of hydrogen or one-half mole of oxygen. The equivalent weight is equal to the atomic weight divided by the valence.

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

• noun chemistry The atomic weight of an element or radical divided by its valence; the molecular weight of a compound divided by its combining power in a specific reaction.

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

• noun the atomic weight of an element that has the same combining capacity as a given weight of another element; the standard is 8 for oxygen