ant·ac·id (?nt-?s??d)adj. Counteracting or neutralizing acidity, especially of the stomach.n. A substance, such as magnesia or sodium bicarbonate, that neutralizes acid.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.antacid (ænt?æs?d) n (Pharmacology) a substance used to neutralize acidity, esp in the stomachadj (Pharmacology) having the properties of this substance: antacid tablets. Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014ant?ac?id (ænt?æs ?d) adj. 1. preventing, neutralizing, or counteracting acidity, as of the stomach. n. 2. an antacid agent. [1725?35] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.