throm·bo·plas·tin (thr?m?b?-pl?s?t?n)n. A complex of protein and phospholipids that is found in tissues and platelets and facilitates blood clotting by converting prothrombin to thrombin.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.thromboplastin (??r?mb???plæst?n) n (Biochemistry) any of a group of substances that are liberated from damaged blood platelets and other tissues and convert prothrombin to thrombin. Also called: thrombokinase Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014throm?bo?plas?tin (??r?m b??plæs t?n) n. a lipoprotein in the blood that converts prothrombin to thrombin. Also called throm?bo?ki?nase (??r?m bo??ka? ne?s, -?k?n e?s) [1910?15]Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.