In evolutionary terms, what does "fitness" refer to?

The concept of "fitness" in evolutionary biology specifically refers to an organism's ability to reproduce and pass on its genes to the next generation. This definition encompasses not just the total number of offspring produced, but also the quality and viability of those offspring, as well as the organism's success in surviving and thriving in its environment long enough to reproduce.

While factors like strength, survival skills, or the ability to evade predators may contribute to an organism's overall success in a particular environment, they are ultimately means to an end— that end being the successful reproduction and transmission of genetic material. Thus, the reproductive success and genetic contribution to subsequent generations are key components in determining evolutionary fitness.