hydrogen ionn. The positively charged ion of hydrogen, H+, formed by removal of the electron from atomic hydrogen and found in the form of hydronium ion in all aqueous solutions of acids.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.hydrogen ion n 1. (Elements & Compounds) an ionized hydrogen atom, occurring in plasmas and in aqueous solutions of acids, in which it is solvated by one or more water molecules; proton. Formula: H+ 2. (Elements & Compounds) an ionized hydrogen molecule; hydrogen molecular ion. Formula: H2+ Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014hy?drogen i?on n. ionized hydrogen of the form H + , found in aqueous solutions of all acids. [1895?1900] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.