For any $i$ and $j$, we write
\[\delta_{i,j} = \begin{cases} 1, & \text{if $i=j$}, \\ 0, & \text{if $i\neq j$}, \end{cases}\]where the meaning of $1$ and $0$ should be clear in the given context.
Using Iverson Brackets we have
\[\delta_{i,j} = [i=j].\]