emergency roomn. Abbr. ER The section of a health care facility for providing rapid treatment to victims of sudden illness or trauma.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.emer?gency room` n. a hospital area equipped and staffed for the prompt treatment of acute illness, trauma, or other medical emergencies. Abbr.: ER Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.