Currently, I define "exercise" in the following way: deliberate physical effort aimed intentionally at improving one or more physical functions of the body, independent of the ability of that body to perform any particular physical function that involves the performance of external tasks.
There is a lot in there. All of it is aimed at keeping exercise exclusive to what lies inside of you versus what you are able to do with it.
Make no mistake: what you are able to do with your body matters. Arguably, it matters the most. The point here, however, is to aim the view of exercise inward. In my experience, that can ironically produce great outcomes in the performance of external tasks.