What does evolution mean in science?

Evolution is the gradual change of a population in response to changes in the environment. Evolution does not occur in individuals, it can only occur when a mutation is added to a population or a certain gene is selected for. Evolution occurs because the fittest individuals who pass their genes on the most are the ones best suited for the environment. Those that don't survive don't pass on their traits. It is important to note that fitness described in survival of the fittest pertains to reproductive fitness, not physical fitness.