What does the forward slash mean set notation mean?

Question

I have an equation that looks like this:

$L(\hat{x}_i, y_i, S) =\begin{cases} \hat{x}_i - y_i & \text{if $i \in S1$} \\ \infty & \text{if $i \in S / \{S2,S3\}$} \end{cases}$

What does this mean

$i \in S / \{S2,S3\}$