In terms of evolution, what does the term "fitness" refer to?

In the context of evolution, "fitness" specifically refers to the ability of an organism to survive and reproduce successfully in its environment. This concept encompasses not just the physical attributes of the organism but also how well suited the organism is to its ecological niche. A higher fitness level means greater reproductive success, which allows the organism to pass on its genes to the next generation. Therefore, a key aspect of evolutionary theory is understanding that fitness is measured by the quantity and viability of offspring rather than physical strength, growth rate, or diet quality. All these factors might play roles in an organism’s fitness, but the primary definition focuses on reproductive success.