ep·i·mer (?p??-m?r)n. One of two molecules that differ in the spatial arrangement of atoms or groups surrounding only one of two or more chiral atoms in the structure.[epi- + -mer, isomer.]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.epimer (??p?m??r?z?m) chemn (Chemistry) either of two optical isomers differing from each other only in the configuration of the atoms about one asymmetric carbon atomn (Chemistry) either of two isomers differentiated by the position of the atoms around a single asymmetric carbon atomCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014