hydrochlorothiazide /ˌhaɪ.dɹəˌklɔɹ.əˈθaɪ.əˌzaɪd/ noun1 •A diuretic drug, with the chemical formula C₇H₈ClN₃O₄S₂, that inhibits the kidneys' ability to retain water and lowers blood pressure. formshydrochlorothiazides