cold fusionn. A hypothetical form of nuclear fusion occurring without the use of extreme temperature or pressure.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.cold? fu?sion n. a hypothetical form of nuclear fusion postulated to occur at relatively low temperatures and pressures, as at room temperature and at one atmosphere. [1980?85] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.