O-ring (??r?ng?)n. A flat ring made of rubber or plastic, used as a gasket.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.O-ring n (General Engineering) a rubber ring used in machinery as a seal against oil, air, etc Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014O-ring (?o??r??) n. a ring of pliable material, as rubber or neoprene, used as a gasket. [1945?50] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.