The term characteristic length or scale refers, in general, to the parameter which characterizes the density profile (of a given physical quantity). The static (equilibrium) or dynamic character of a characteristic length must be specified. The terms out of plane and in plane refer to characteristic lengths normal or parallel to the interface, respectively. Since interfacial layer 'thickness' and characteristic length correspond to various concepts, the current usage where an out of plane characteristic length is referred to as the interfacial layer thickness, is confusing and should be abandoned.