"$\backslash$" notation question in definition for Nash equilibrium A definition in my book states


  $ \sigma^* \in \Delta$ is a Nash equilibrium for $\Gamma$ if for all $i \in I$ and all $\mu_i \in \Delta_i, \; U_i (\sigma^*) \geq U_i(\sigma^* \backslash \mu_i)$.  The set of Nash equilibria for $\Gamma$ is denoted $Eq(\Gamma)$


What does the $\backslash$ here represent, divide or element not included?
Informally, a (possibly mixed) set of strategy choices is a Nash equilibrium if no player can expect to benefit by changing strategies while the other players keep their strategies unchanged. 

Here I would interpret your definition as $\Delta$ being the set of all possible strategy choices across all players, $\sigma^*$ being a set of strategy choices which will be a Nash equilibrium if it satisfies the conditions, $i$ being a player, and $\sigma^* \backslash \mu_i$ being a change in player $i$'s strategy while the other players' strategies stay the same.  So I would take $\backslash$ having a type of "not included" interpretation.