Fitness is best defined as increased work capacity across broad time, modal, and age domains. This means is that a fit individual has functional capacity in all types of movements at a variety of durations of effort, throughout the duration of life. 
Prior to this definition, fitness was commonly defined as good health or physical condition, especially as the result of exercise and proper nutrition. It is also known to be taken as the state or condition of being fit, or in shape. When the term good health is taken into account, it implies that a "fit" person is essentially the polar opposite of one who is ill. If this logic is taken even further, it stands to reason that there are varying degrees of fitness, with illness being at one end of the spectrum, and elite athleticism being at the other end.
Components of Fitness
Physical fitness is made up of many components (in alphabetical order):
It is generally accepted that a person who exemplifies "fitness" possesses all of these qualities.