on·dan·se·tron (?n-d?n?s?-tr?n?)n. A selective serotonin receptor antagonist, C18H19N3O, used in its hydrochloride form to prevent nausea and vomiting, especially when occurring postoperatively or as a result of chemotherapy or radiotherapy.[ondan-, of unknown origin + -setron (from (se(ro)t(onin) r(eceptor antag)on(ist)).]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.