Fitness, in the most broad terms, is the ability to perform the tasks of life. The tasks of life are always some form of moving your own body and moving external loads. These tasks can be measured using the fundamental units of force, time, and distance. Force (your bodyweight or the weight of the object you are moving), to be measured by a scale. Multiplied by Distance (the distance your center of mass moves or the distance the object moves, across the number of reps performed), measured with a measuring tape. Divided by Time (how long did it take you to complete your given task) measured with a stop watch. Giving us average Power output.
More precisely, having elevated levels of work capacity (power output ability) across broad time (short, medium, and long durations) and modal domains (multiple skills, drills, and tasks) is fitness. At any and every stage of life, being sick limits your work capacity. In contrast, having this broad work capacity requires you to be healthy.
How does this relate to the traditional markers of health, such as blood pressure, triglycerides, and cholesterol? Well, they are correlates, just as VO2 Max is a correlate to fitness. Would you give up mobility and work capacity in order to lower your blood pressure? Of course not! You want to lower your blood pressure in order to maintain your mobility and physical freedom.
CrossFit’s definition of health is not designed to support the results of CrossFit’s workouts. The definition of health is an independent marker that allows us to measure the efficacy of all fitness and wellness programs, not just CrossFit.