Atenolol: A medication that blocks the action of a portion of the involuntary nervous system that stimulates the pace of the heartbeat. By blocking the action of these nerves, atenolol reduces the heart rate and is useful in treating abnormally rapid heart rhythms. Atenolol also reduces the force of heart muscle contraction, lowers blood pressure, and is helpful in treating angina. It is also used for the prevention of migraine headaches and the treatment of certain types of tremors. The brand name is Tenormin. Generic is available.