EVOLUTIONARILY STABLE STRATEGY. A strategy that cannot be displaced by any other known strategy. Put another way, when its frequency is 1.0 other strategies cannot enter the population and in situations where it appears as a mutant, it increases to fixation. It is a static condition. ESS's can come in two general types -- pure and mixed.
Evolutionarily stable strategies. A heritable (commonly but by no means always behavioral) strategy which, if adopted by (expressed in) most members of a population, cannot be supplanted in evolution by an alternative (mutant) strategy.
Evolutionary Stable Strategy. In game theory and biology, a strategy that, when possessed by an entire population, results in an equilibrium such that mutation of the strategy can never result in an improvement for an individual. Always Defect is an ESS, while Always Cooperate is not.