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Fitness may means:
Physical fitness, a general state of good somatic health and abilities, usually as a result of exercise and nutrition
Fitness competition, a form of physique competition for women, related to bodybuilding
Fitness (biology), an individual’s ability to propagate its genes
In mathematics and computer science, the degree to which a given function is optimized; see optimization

Physical fitness is an attribute required for service in virtually all militaries.The notion of physical fitness is used in two close meanings.

General fitness
In its most general meaning, physical fitness is a general state of good physical health. A person with a physical impairment may be physically fit and healthy, though their performance on tasks requiring full bodily function in the area of impairment will be affected.

Physical fitness is a result of regular physical activity, proper diet and nutrition, and proper rest for physical recovery within the parameters allowed by the genome.

Physical fitness is often divided into following types:

Many sources also cite mental and emotional health as an important part of overall fitness. This is often presented in ****books as a triangle made up of three sub-sections which represent physical, emotional, and mental fitness. Hence, one may be physically fit but may still suffer from a mental illness or have emotional problems. The "ideal triangle" is balanced in all areas.

Task-oriented fitness
A human may be said to be physically fit to perform a particular task with a reasonable efficiency, for example, fit for be military service. Fitness testing is conducted for fire fighters and police officers to determine if they are capable of the physically demanding tasks required for the job before they are employed. This type of fitness testing is a way for employers to discriminate against those incapable of performing the required tasks without discriminating based on gender or disability.

Military-style
In recent years, Military-style fitness training programs have become increasingly popular among civilians. Courses are available all over the U.S. and Europe.

They are usually taught by ex-military personnel. Very often the instructors held highly regarded positions within various military organizations. Oftentimes the instructors were formerly Drill instructors, Special Forces Operatives or held otherwise distinguished positions.

These courses always have some common elements. They often focus on military style calisthenics and group runs. The courses are often held very early in the morning and will meet in almost any weather. Students can expect push-ups, sit-ups, pullups, and jumping jacks, as well as more obscure drills such as flutter kicks, sun worshippers and flares. Almost invariably a workout will include short runs while longer runs are more scheduled”. Special forces are renowned for their level of fitness and intensity of their workouts.

One such example is Crossfit, which is popular with military and law enforcement personnel
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Fitness (biology)

Fitness (often denoted w in population genetics models) is a central concept in evolutionary theory. It describes the capability of an individual of certain genotype to reproduce, and usually is equal to the proportion of the individual’s genes in all the genes of the next generation. If differences in individual genotypes affect fitness, then the frequencies of the genotypes will change over generations; the genotypes with higher fitness become more common. This process is called natural selection.

Measures of fitness
There are two commonly used measures of fitness; absolute fitness and relative fitness.

Absolute fitness (wabs) of a genotype is defined as the ratio between the number of individuals with that genotype after selection to those before selection. It is calculated for a single generation and may be calculated from absolute numbers or from frequencies. When the fitness is larger than one, the genotype increases in frequency; a ratio smaller than one indicates a decrease in frequency.
Absolute fitness for a genotype can also be calculated as the product of the proportion survival times the average fecundity.

Relative fitness is quantified as the average number of surviving progeny of a particular genotype compared with average number of surviving progeny of competing genotypes after a single generation, i.e. one genotype is normalized at w = 1 and the fitnesses of other genotypes are measured with respect to that genotype. Relative fitness can therefore take any nonnegative value, including 0.

While researchers can usually measure relative fitness, absolute fitness is more difficult. It is often difficult to determine how many individuals of a genotype there were immediately after reproduction.

The two concepts are related, and **** of them are *****alent when they are divided by the mean fitness, which is weighted by genotype frequencies.

Because fitness is a coefficient, and a variable may be multiplied by it several times, biologists may work with "log fitness" (particularly so before the advent of computers). By taking the logarithm of fitness each term may be added rather than multiplied.

Discussion
An individual’s fitness is manifested through its phenotype. As phenotype is affected by **** genes and environment, the fitness levels of different individuals with same genotype are not necessarily equal, but depend on the environment in which the individuals live.

As fitness measures the quantity of the copies of the genes of an individual in the next generation, it doesn’t really matter how the genes arrive in the next generation. That is, for an individual it is equally "beneficial" to reproduce itself, or to help relatives with similar genes to reproduce, as long as similar amount of copies of individual’s genes get passed on to the next generation. Selection which promotes this kind of helper behaviour is called kin selection.

A fitness landscape is a way of visualising fitness in terms of peaks, where natural selection will always push uphill but only locally, resulting in suboptimality.

Where there are differences in fitness, a genetic load is exerted on the population.

Richard Dawkins introduced the controversial concept of ethical fitnessism.

History
The British sociologist Herbert Spencer coined the phrase "survival of the fittest" (though originally, and perhaps more accurately, "survival of the best fitted") in his 1851 work Social Statics and later used it to characterise what Charles Darwin had called natural selection. The British biologist J.B.S. Haldane was the first to quantify fitness, in terms of the modern evolutionary synthesis of Darwinism and Mendelian genetics starting with his 1924 paper A Mathematical Theory of Natural and Artificial Selection. The next further advance was the introduction of the concept of inclusive fitness by the British biologist W.D. Hamilton in 1964 in his paper on The Evolution of Social Behavior