zo·o·plank·ter (z???-pl?ngk?t?r)n. One of the animal organisms constituting zooplankton.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.zooplankter (?z????plæ?kt?) n (Biology) one of the animal organisms making up zooplanktonCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014