Distinguished Conduct Medaln. Abbr. DCM A British military decoration for distinguished conduct in the field.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.Distin?guished Con?duct Med`al n. a British military decoration awarded for distinguished conduct in operations in the field against an enemy. Abbr.: D.C.M. Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.