frobenius norm

Frobenius norm

$$\begin{array}{r}\Vert A{\Vert }_F=\sqrt{\sum _{i,j}|A_{ij}{|}^2}\end{array}$$

For non complex matrices, you don’t need to take the absolute value.
If you turn a matrix into a big vector, the frobenius norm is just the L2 norm of that vector.