(2) The capability to perform a function based on its suitability
In general, fitness pertains to the state or quality of being fit, e.g. physically or mentally, as in physical fitness. In biology though, "fitness" is more encompassing. It is concerned more on the reproductive capability of an organism and its genetic contribution to the gene pool. Fitness, in biological context, is also called Darwinian fitness as it relates to Charles Darwin's evolutionary theory on natural selection. Darwinian fitness describes how successful an organism has been at passing on its genes. The more likely that an individual is able to survive and live longer to reproduce, the higher is the fitness of that individual. Thus, it may indicate the relative measure of reproductive success of an organism in passing its genes to the next generation. It denotes the relative ability of an individual (or population) to survive, reproduce and propagate genes in an environment. There are two ways through which fitness can be measured: (1) absolute fitness and (2) relative fitness.