It always amazes me that FITNESS is a multi-billion dollar industry... Yet a great, concise and universal definition of fitness is hard to nail down.
Just ask anybody participating in a physical pre workout supplement bcaa (link web page) plan, as well as someone in the health industry for that matter, to give you an obvious definition of health and fitness.
You will most likely be welcomed with the "deer found in the headlights" stare accompanied by nonsensical stammering.
If you do get an answer, it'll more than likely be slanted toward the specific capabilities of the person asked.
For example...

A marathon runner is going to define fitness in terms of muscular and cardiorespiratory endurance... A power-lifter is going to define fitness in conditions of absolute strength... A bodybuilder will define fitness in terms of muscle size as well as definition... etc.
But the vast majority of people participating in a physical exercise routine are not professional athletes worried about the improvement of one physical ability.
Therefore, basing your definition of fitness on a single certain physical ability does not result in a universal definition of health.