Does the expression "stall speed" have a definition?
Indeed, "the stall speed of an airfoil doesn't exist." But the stall speed of an airplane does exist, though it's dependent on the aircraft weight $W$, load factor $n_z$, configuration (e.g., flaps position) and the air density. This variability makes it disadvantageous (for pilots---even dangerous) to think of an airplane's stall in terms of a definite stall speed $v_{Stall}$, as if it were a constant characteristic of the type. What are (roughly) constant are the stall angle of attack and the stall lift coefficient $c_{L_{Stall}}$.

The equilibrium equation for the forces acting on the airplane In stall in its symmetry plane and perpendicular to its velocity is:

$$n_z\cdot W = \frac{1}{2}\cdot\rho\cdot v^2_{Stall}\cdot S_{ref}\cdot c_{L_{Stall}}$$

Doing the algebra, one can easily exspress the stall speed in terms of the remaining parameters ($\rho$ = density of air, $S_{ref}$ = reference area) The resulting expression will explain (i) that for an airplane, stall speed does exist; (ii) the nature of its dependences and (iii) that for an airfoil, to which force equilibrium does not apply, there is no "stall speed."