Not sure about if being now but the definition is the state or quality of being fit.
I do whatever keeps my body in shape. Fitness is known as the quality of being fit and being in that state of mind.
Hey do you go to the gym? I do not, did you know that "fitness" meant being able to do the day's stuff without being excessively tired? This was before the Industrial Revolution. That's interesting! What does it mean nowadays?
Fitness is defined as the quality or state of being fit.