Weak evolutionarily stable strategy
A weak evolutionarily stable strategy is a more broad form of evolutionarily stable strategy. Like ESS, a WESS is able to defend against an invading "mutant" strategy. This means the WESS cannot be entirely eliminated from the population.
The definition of WESS is similar to ESS. Any strategy s is a weakly evolutionarily stable strategy if for any strategy s*≠s:
u > u or
u = u and u ≥ u.
One example of WESS, in a prisoner's dilemma, is Tit-for-tat.