  The conventional classification of electrolyte solutions as 'strong' or
'weak' accounts for their charge transport properties, but neglects their mass
transport properties, and is not readily applicable to highly concentrated
solutions. Here, we use the Onsager transport formalism in combination with
linear response theory to attain a more general classification taking into
account both charge and mass transport properties. To this end, we define a
molar mass transport coefficient {\Lambda}_mass, which is related to
equilibrium center-of-mass fluctuations of the mobile ions and which is the
mass-transport analogue of the molar ionic conductivity {\Lambda}_charge. Three
classes of electrolyte solution are then distinguished: (i) 'Strong'
electrolytes with 4{\Lambda}_mass \approx {\Lambda}_charge; (ii) 'weak charge
transport electrolytes' with {\Lambda}_charge << 4{\Lambda}_mass; and (iii)
'weak mass transport electrolytes' with 4{\Lambda}_mass << {\Lambda}_charge.
While classes (i) and (ii) encompass the classical 'strong' and 'weak'
electrolytes, respectively, many highly concentrated electrolytes fall into
class (iii) and thus exhibit transport properties clearly distinct from
classical 'strong' and 'weak' electrolytes.
