Cardio is short for cardiorespiratory or cardiovascular exercise and refers to exercise that elevates the heart rate to pump oxygen and nutrient-carrying blood to the working muscles. Most often used for exercise performed on equipment like treadmills, elliptical runners or stationary bikes, it is important to know that ANY exercise that elevates the heart rate can provide cardiorespiratory benefits. Circuit training with free-weights or performing an AMRAP (as many rounds of a particular circuit as possible in a given amount of time) can be considered cardiorespiratory exercise.