Mesh-size definition

Hello,

I couldn't find the exact definition of the meshsize parameter maxh for 2D/3D geometries.
Is it 'just' the maximum length of one edge of the element? Or is it the radius/diameter of a circumball, in-ball or the min-containment ball?

Thanks in advance for your input. 

it is the maximal edge-length

it is a recommendation, not a hard limit