(4) The state or quality of being fit, e.g. physically or mentally; being in shape, good health or well-being.
(5) The capability to perform a function.
(6) The ability of an organism to survive and reproduce in its enviornment.
In biology, Darwinian fitness or simply fitness of a biological trait 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.
There are two ways through which fitness can be measured: