Alpha blocker: A drug that blocks receptors in arteries and smooth muscle. This action relaxes the blood vessels and leads to an increase in blood flow and a lower pressure for the control of hypertension. The action in the urinary tract enhances urinary flow in prostatic hypertrophy (enlarged prostate). The alpha blockers include doxazosin (Cardura), prazosin (Minipress), and The alpha blockers include doxazosin (Cardura), prazosin (Minipress), and terazosin (Hytrin).