Health: Easy enough. Normally-to-excellently functioning internal organs, reflexes, immune system; healthy skin, eyes, etc.
Basically, you are in good working order and can fight off sickness well.
Fitness: Also easy, though not as straightforward. The basic level of human functionality would be the ability to perform regular, daily tasks without breaking down. However, our "daily tasks" have changed dramatically from even 100 years ago as compared to most people's cushy existence today.
Therefore, fitness should be defined, to my mind, as a combination of various strengths. I would not consider a strong-but-obese man to be "fit," nor would I consider a paper-weight, 95lb weakling who couldn't squat his own bodyweight "fit."
Fitness is a combination of good levels of strength, stamina, agility, and speed.
Form follows function.