Definition. A strategy \(\sigma\) has a uniform invasion barrier if there exists an \(\bar\epsilon \gt 0\) such that for all \(\mu \neq \sigma\) and \(\epsilon \in (0,\bar\epsilon)\), \[ \pi( \sigma \mid \epsilon \mu + (1-\epsilon)\sigma) \gt \pi( \mu \mid \epsilon \mu + (1-\epsilon) \sigma). \]