ammonium hydroxiden. A colorless, basic, aqueous solution of ammonia, NH4OH, used as a household cleanser and in the manufacture of a wide variety of products, including textiles, rayon, rubber, fertilizer, and plastic. Also called ammonia, ammonia water.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.ammonium hydroxide n (Elements & Compounds) a compound existing only in aqueous solution, formed when ammonia dissolves in water to form ammonium ions and hydroxide ions. Formula: NH4OH. Also called: aqueous ammonia Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014ammo?nium hydrox?ide n. a basic liquid compound, NH4OH; ammonia water. [1900?05] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.