VO2 Oxygen

This is the maximum amount of oxygen (Volume of Oxygen) the body can use during a period of intense exercise that largely depends on body weight and the strength of the lungs. It’s the point during an exercise where intensity level can continue to rise, but the amount of oxygen you’re taking in does not increase with it. VO2 max typically declines with age, but the better shape your in, the higher it can be.

Some consider this the best metric of aerobic fitness because it specifically shows how well your body is able to use oxygen.