reducing agent

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

• noun A substance that chemically reduces other substances, especially by donating one or more electrons.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Chem.) a substance that causes reduction of another substance in a chemical reaction, as by donating electrons or adding hydrogen atoms.

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

• noun chemistry Any substance that reduces, or donates electrons to, another; in so doing, it becomes oxidized.

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

• noun a substance capable of bringing about the reduction of another substance as it itself is oxidized; used in photography to lessen the density of a negative or print by oxidizing some of the loose silver