zy·go·sis (z?-g??s?s, z?-)n. pl. zy·go·ses (-s?z) The union of gametes to form a zygote; conjugation.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.zygosis (za?????s?s; z?-) n (Biology) biology another name for conjugation zygose adjCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014zygosisthe biological process of conjugation; the union of cells or gametes. ? zygose, adj.See also: Biology-Ologies & -Isms. Copyright 2008 The Gale Group, Inc. All rights reserved.