What does the notation $N_{i;j}$ mean in this numerical relativity / differential equations context here?
$$ \frac{d}{dt}g_{ij} = \frac{2N}{\sqrt g}(\pi_{ij} - \frac{1}{2}\pi g_{ij}) + N_{\color{red}{i;j}} + N_{\color{red}{j;i}}$$
where $N_i = g_{0i}$