phe·no·thi·a·zine (f??n?-th???-z?n?, -n?-)n.1. A yellow organic compound, C12H9NS, used in insecticides, livestock anthelmintics, and dyes.2. Any of a group of drugs derived from this compound and used in the treatment of psychiatric disorders, such as schizophrenia.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.phenothiazine (?fi?n????a??zi?n) n1. (Elements & Compounds) a colourless to light yellow insoluble crystalline compound used as an anthelmintic for livestock and in insecticides. Formula: C12H9NS2. (Pharmacology) any of several drugs derived from phenothiazine and used as strong tranquillizers and in the treatment of schizophreniaCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014phe?no?thi?a?zine (?fi n???a? ??zin, -z?n) n. 1. a grayish green to greenish yellow, crystalline, water-insoluble solid, C12H9NS, used as an insecticide and vermifuge and in the synthesis of pharmaceuticals. 2. any of a class of medications used principally to treat psychotic symptoms, as hallucinations, and excessive excitability. [1890?95] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.