An evolutionary stable strategy (ESS) is a concept in evolutionary game theory that describes a strategy which, if adopted by a population, cannot be invaded by any alternative strategy. This means that if most individuals in a population use this strategy, any mutant strategy that arises will not be able to outperform it, ensuring the original strategy's persistence over time. ESS is often used to explain behaviors in animals, such as hawk-dove games, where individuals adopt aggressive or peaceful strategies in resource competition. The concept helps researchers understand how certain traits or behaviors become dominant in a population through natural selection and evolutionary processes.