 Definition. A strategy \(\sigma\) is an evolutionarily stable strategy (ESS) if and only if for all other strategies \(\mu \neq \sigma\) it is the case that either \(\pi(\sigma \mid \sigma) \gt \pi(\mu \mid \sigma)\) or that \(\pi(\sigma \mid \sigma) = \pi(\mu \mid \sigma)\) and \(\pi(\sigma \mid \mu) \gt \pi(\mu \mid \mu)\).