What is the stall speed of an aircraft?

Stalls occur when a wing's angle of attack is too large, surpassing some critical 'stall angle'. At this point, flow separates and the pressure differential on the bottom and top surfaces fails. Lift goes with it.

For a given weight (plane and cargo), the wings can provide sufficient lift through infinite combinations of angle of attack and velocity. As one goes down, the other must go up to compensate.

Since 'stall angle' goes down with velocity, however, there is a problem. There is some minimum velocity at which the angle of attack must be greater than 'stall angle' in order to produce lift, but being past this angle, it stalls and lift fails anyway. This velocity is called 'stall speed'.

Thus 'stall speed' is the absolute minimum speed a plane can fly at.