Fitness is a technical term from population genetics
Population geneticists have adopted the term fitness largely unchanged. In order to be able to measure fitness in a meaningful way, the term was also clarified somewhat and a description in mathematical language was justified. This was particularly necessary in order to be able to grasp it not only as a property of individual individuals but also of populations. The arithmetic mean and variance of the population can be calculated.
The mean absolute fitness corresponds exactly to the mean individual fitness, since in the second case the same individuals have only been divided into two or more subgroups. Absolute fitness values are usually not very useful for processing, as they do not allow comparisons. Therefore, the absolute fitness value is usually normalized to a reference value.