The $n\times n$ identity matrix $I_n$ is the $n\times n$ matrix whose entries $(a_{ij})$ are such that $a_{ij}=1$ when $i=j$ and $0$ otherwise. That is, $a_{ij} = \delta_{ij}$ where $\delta_{ij}$ is the Kronecker delta.
Wikidata ID: Q193794