Set of "rationalizable actions"

A strategy is rationalizable if it survives an iterated elimination of strictly dominated strategies. That is first you eliminate all strategies that are never a best response independent what action the other player chooses. Then you only consider the strategies that survive and repeat this step: eliminate all strategies that are never a best response independent what action from the set of surviving strategies the other player chooses.

The set of rationalizable strategies is then a set that only contains best responses to some rationalizable strategy. Therefore this is a larger set than Nash equilibria, which are best responses against each other.