They say that if you took twenty different types of sports you will get twenty different types of fitness definitions from the participants
. It does not make any difference whether you are involved with anything from Yoga to surfing it will have its own specific type of fitness that it needs.
The dictionary describes fitness as the ability of something or someone to adapt to the environment. When we are intent on getting fit or improving our fitness it is all relative to how fit we currently are and for what specific sport or activity we would want to get fit. The training that you do will be in order to increase your fitness and ability to adapt.
But physiologically speaking there is only one definition that is needed to describe fitness and sports science who studies this have come up with a very specific way to establish this fitness. They do it by loading the body as in running or cycling or even rowing and they then measure the amount of air that is needed to be processed.
This is called VO2 Max and is basically the workload divided by the amount of air that is required in order to process that workload. Whatever the reading is it should be understood that the reading is specific to the time and place that it was done. This means that you are only as fit as your last workout.
Because your fitness is your physical ability to adapt to the environmental stress that you are under, the more stress that you are given the more your body will adapt. This means that someone who jogs a mile a day and does the same thing every day at the same pace they will not be getting fitter.
They will improve their fitness to a pointy where the body will be able to do the required mile job within the given time allocated and if the person does not run faster the next day or run farther then the fitness that they have achieved will not be increasing. It will increase to the point that the person will be able to do the mile jog without tiring.
But this does not mean that the time and energy that it takes to jog a mile is a waste of time. On the contrary it is very beneficial even if the fitness is not increased because it will have all the physiological benefits of getting to that level of fitness. This means that any exercise is good exercise because it will be keeping you healthy.