magnesium hydroxide

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

• noun A white powder, Mg(OH)₂, used as an antacid and laxative.

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

• noun (Chem.) A slightly alkaline chemical substance, Mg(OH)2, the active principle in the antacid milk of magnesia, also used as a laxative.

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

• noun inorganic chemistry The base Mg(OH)2 that occurs naturally as brucite; it is used as an antacid and in many industrial processes

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

• noun a white crystalline powder used chiefly in medicines