exists and is an alternate of . In biological terms, fitness is the suitability of an individual, or of a species, for its environment: the better-adapted it is, the more likely it is to survive, and to thrive. In other words, it fits. fitness: the ability of an organism to survive and reproduce in its environment. In biological terms, <i>fitness</i> is the <b>suitability of an individual, or of a species, for its environment</b>: the better-adapted it is, the more likely it is to survive, and to thrive.