o·o·cyst (???-s?st?)n. A thick-walled structure that contains the zygote of an apicomplexan parasite and releases the infective sporozoites.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.oocyst (?????s?st) n (Zoology) an encysted zygote of sporozoan protozoans that undergoes sporogony to produce infective sporozoitesCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014o?o?cyst (?o? ??s?st) n. the encysted zygotic stage in the life cycle of some sporozoans. [1870?75] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.