pos·i·tro·ni·um (p?z??-tr??n?-?m)n. A short-lived association of an electron and a positron bound together in a configuration resembling the hydrogen atom.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.positronium (?p?z??tr??n??m) n (Atomic Physics) physics a short-lived entity consisting of a positron and an electron bound together. It decays by annihilation to produce two or three photons[C20: from positron + -ium]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014pos?i?tro?ni?um (?p?z ??tro? ni ?m) n. a short-lived atomic system consisting of a positron and an electron bound together. [1945; positron + -ium2] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.