Even though fitness is a multi-billion dollar industry today, it still lacks a solid and concise universal definition. If you ask someone from the fitness industry or a person participating in a physical fitness program, they will not be able to give you a clear definition. In fact the most you will be able to get out of them is an answer slanted towards their specific ability. Like a power lifter will define fitness in terms of strength, a marathon runner will define it in terms of cardiorespiratory endurance and a body builder will define it in terms of muscle size.
The problem is that everyone participating in a physical fitness program is not an athlete and therefore is not concerned with improving just that one specific physical ability. Hence basing the definition of fitness on that specific physical ability can’t really lead us to a universal definition. Getting a clear idea about what fitness really means is an integral part of achieving physical fitness. So for a person who wants to be a better all-around human being should know that fitness relates to the degree of competence and skill required to take part in all of the physical activities including strength, cardiorespiratory endurance, power, toughness, flexibility, speed, agility accuracy, toughness and balance.
Some people will criticize this definition by saying that you cannot maximize all of the performance abilities at the same time. While they are right in their own place but you should know that fitness is not about maximizing just one specific physical set at a time but it relates to the optimization of all the physical skills at any given time.