Diuretic: Something that promotes the formation of urine by the kidney. All diuretics cause a person to 'lose water,' but they do so by diverse means, including inhibiting the kidney's ability to reabsorb sodium, thus enhancing the loss of sodium and consequently water in the urine (loop diuretic); enhancing the excretion of both sodium and chloride in the urine so that water is excreted with them (thiazide diuretic); or blocking the exchange of sodium for potassium, resulting in excretion of sodium and potassium but relatively little loss of potassium (potassium-sparing diuretic). Some diuretics work by yet other mechanisms, and some have other effects and uses, such as in treating hypertension. Also known as water pill. Substances in food and drinks, such as coffee, tea, and alcoholic beverages, may act as diuretics.