Evolutionarily stable strategy

In game theory and behavioural ecology, an evolutionarily stable strategy (or ESS; also evolutionary stable strategy) is a strategy which, if adopted by a population of players, cannot be invaded by any alternative strategy. An ESS is an equilibrium refinement to a Nash equilibrium. An ESS is a Nash equilibrium which is "evolutionarily" stable meaning that once it is fixed in a population, natural selection alone is sufficient to prevent mutant strategies from successfully invading.
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(ESS): A strategy such that, if most members of a population adopt it, would give a reproductive fitness higher than any mutant strategy.