Fitness is defined as the quality of be

Fitness is defined as the quality of being suitable to perform a particular task. Around 1950, perhaps consistent with the industrial revolution and the treatise of WWII, the term fitness increased in western vernacular by a factor of ten.[2] Modern definition of fitness describe either a person or machine's ability to perform a specific function or a holistic definition of human adaptability to cope with various situations. This has lead to an interrelation of human fitness and attractiveness which has mobilized global fitness and fitness equipment industries. Regarding specific function, fitness is attributed to personnel who possess significant aerobic or anaerobic ability, i.e. strength or endurance. A holistic definition of fitness is described by Greg Glassman in the CrossFitt journal as an increased work capacity across broad times and model domains; mastery of several attributes of fitness including strength, endurance, power, speed, balance and coordination and being able to improve the amount of work done in a given time with any of these domains.[3] A well rounded fitness program will improve a person in all aspects of fitness, rather than one, such as only cardio/respiratory endurance or only weight training.

A U.S. Marine performing a fitness routine.
A comprehensive fitness program tailored to an individual typically focuses on one or more specific skills,[4] and on age-[5] or health-related needs such as bone health.[6] Many sources[7] also cite mental, social and emotional health as an important part of overall fitness. This is often presented in textbooks as a triangle made up of three points, which represent physical, emotional, and mental fitness. Physical fitness can also prevent or treat many chronic health conditions brought on by unhealthy lifestyle or aging.[8] Working out can also help people sleep better and have a better mood. To stay healthy it is important to engage in physical activity.[9]
The U.S. Centers for Disease Control and