NAD (?n??-d??)n. A coenzyme, C21H27N7O14P2, occurring in most living cells and utilized alternately with NADH as an oxidizing or reducing agent in various metabolic processes.[n(icotinamide) a(denine) d(inucleotide).]nad (n?d)n. Vulgar Slang A testicle.[Short for gonad.]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.NAD n (Biochemistry) biochem nicotinamide adenine dinucleotide; a coenzyme that is a hydrogen carrier in metabolic reactions, esp in tissue respiration. Former name: DPN Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014NAD nicotinamide adenine dinucleotide: a coenzyme, C21H27N7O14P2, involved in many cellular oxidation-reduction reactions. Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.