The term "fittest" is used in Charles Darwin's theory of evolution by natural selection. According to this theory, individuals within a species that possess traits best suited for their environment are more likely to survive and reproduce than those without these advantageous traits. Therefore, the fittest individuals will pass on their genes to future generations in greater numbers, leading to an increase in the frequency of those beneficial traits throughout the population over time. In essence, being "fittest" means having a combination of physical and behavioral adaptations that allow one to survive and reproduce more successfully than others within their environment.