Wikipedia defines physical fitness as a measure of the human body's ability to function efficiently and effectively in work and leisure activities, to be healthy, to resist hypokinetic diseases, and to meet emergency situations.
I like this definition of physical fitness. It describes what we're trying to achieve in many situations by employing strength training and conditioning activities. I'll say this again and again, John Wooden in my mind was spot-on when he stressed physical fitness and fundamentals of the sport for his athletes. This is as simple as it gets when seeking a mindset for developing athletes.