ce·tir·i·zine (s?-tîr??-z?n?)n. A nonsedating antihistamine, C21H25ClN2O3, used in the form of its hydrochloride to treat allergic rhinitis and other allergic disorders.[(a)cet(ic acid) + alteration of (pip)erazine.]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.