(biology) Adapted, i.e. in a way that an organism is able to live and reproduce offspring in an environment
(general) Suited; suitable; proper; appropriate
To be adapted or suitable
Something that fits
In biology, the term fit pertains to the suitability of an organism to the prevailing conditions of its environment. A living thing that is fit indicates the capacity to live, survive, and pass on the genes to the offspring. A biological condition wherein a competing variant is increasing in frequency relative to other competing variants in a population is called fitness. Darwinian fitness of a biological trait describes how successful an organism has been at passing on its genes. The more likely that an individual is able to survive and live longer to reproduce, the higher is the fitness of that individual.
The phrase "survival of the fittest" relates to the reproductive success of an organism based on the suitability of its genotype and phenotype to its environment. The phrase is coined by Herbert Spencer, who based it on Charles Darwin's evolutionary theory, natural selection.