fitness (biology) – an organism's ability to survive and reproduce within an environment

To remember the meaning of fitness (biology), use the following mnemonic:

The animals did fitness (fitness) classes in order to survive in this environment. 

Fitness refers to an organism's ability to survive and reproduce in its environment, passing on its genes to the next generation. It's not about physical strength or athletic ability (although that's a useful connection to make for memory purposes), but rather how well-adapted an organism is to its specific habitat and how successfully it can produce offspring that also survive to reproduce. An organism with high biological fitness has characteristics or adaptations that help it survive longer, find mates more easily, and produce more offspring that survive to reproductive age. For example, a bird with a beak shape perfectly suited to accessing its food source would have a higher fitness than birds with less suitable beaks. 

The concept is central to natural selection, as organisms with higher fitness are more likely to pass on their advantageous traits to future generations, while those with lower fitness may struggle to survive or reproduce and their genes become less common in the population over time.