You ask my personal understanding of the term 'fitness' so it doesn't matter if this differs from something you'll find in the dictionary. I'd saythat someone is fit if they can run up a flight of stairs without becoming breathless, if their muscles are reasonably toned, their eyes are bright and their minds lively. I dare say that fitness instructors have especial standards by which they judge fitness, or 'the level of fitness'.
It is no good stating the number of sit-ups someone can do, for an elderly person may be fit yet not be able to do any sit-ups at all. Fitness takes account of age, bone age, and perhaps it should include the readiness to engage in suitable activities. i.e. not say 'Oh it's raining, so I will only go out in the car'. If you are young, you should display appropriate energy. Couch potatoes are certainly not my idea of 'fit'