Bicuspid aortic valve: An aortic valve in the heart that has two flaps (cusps) that open and close. A normal aortic valve in the heart has three flaps. There may be no symptoms of bicuspid aortic valve in childhood, but in time the valve may become narrowed, making it harder for blood to pass through it, or blood may start to leak backward through the valve (regurgitate). Treatment depends on how the valve is working. For a severely deteriorated valve, replacement surgery may be necessary.