side chainn. An organic molecular structure in the form of a branched open chain or ring.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.side chain n (Chemistry) chem a group of atoms bound to an atom, usually a carbon, that forms part of a larger chain or ring in a molecule Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014