I recently came across this notation, without explanation, in a paper:

$||\mathbf{W}||_{2,1}$

From the context, I know that $\mathbf{W}$ is a matrix, which could be any size, and that $||\mathbf{W}||_{2,1}$ must be a scalar. Can anyone tell me what this means, or just what it's called, so I can look it up?