Define Fitness? According to Dawin's ideas of evolution, what makes an organism have the greatness fitness?

In biology, fitness can be described as the ability of an organism to survive in a particular environment and to reproduce and pass on its traits to its offsprings.

According to Darwin's idea of evolution, those organisms have greater fitness which are better adapted to live in an environment and which can adapt to changes in an environment. These organisms then pass their characteristics to their offsprings.