This is hands down the best definition of Physical Fitness that I have ever read. I believe that every physical education teacher should have this posted in their gyms, no matter what age they are currently teaching.
Here is the modern definition of physical Fitness:
Physical Fitness is a set of physical attributes that people have of achieve relating to their ability to perform physical activity. These attributes provide the foundation for tasks of daily living, are associated with reduced risk of chronic diseases, and provides a basis for involvement in lifetime physical activity.
This definition was given to me by Dr. Kniffin in my EDU 355 teaching block class.