Does the expression "stall speed" have a definition? I read, not only on this site, that the stall speed of an airfoil doesn't exist, and I usually make the effort to stay away from this expression.

While an airfoil can stall at any airspeed, it's clear that the stall speed refers to the airspeed at which the stall occurs in an horizontal 1G flight.


Stall speeds for B747-200 (n=1). (Source).

This speed is the lowest one that can generate a vertical component of lift equal to the weight with the airfoil at its stall angle of attack.

I would compare this implicit assumption with the maximum airspeed of an aircraft: It's clear we are assuming at least an horizontal flight and a standard atmosphere, etc, else this speed can be anything too.

So what's wrong with this definition of the "stall speed"? Is the stall speed defined in the aerodynamics field?

Example of use: Stall Speed (Dr. Mustafa Cavcar).
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."