Post Exchange Abbr. PX A service mark used for a store on a military base that sells goods to military personnel and their families or to authorized civilians.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.post exchange n (Military) US a government-subsidized shop operated mainly for military personnel. Abbreviation: PX Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014post? exchange` n. a retail store on a military post. [1890?95] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.