hy·dro·chlo·ro·thi·a·zide (h??dr?-klôr??-th???-z?d?)n. A diuretic drug, C7H8ClN3O4S2, used in the treatment of hypertension.[hydro- + chloro- + thiaz(ole) + -ide.]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.