Consider the flight envelope / V-N diagram...
(this is just an example I have "borrowed" from a University website)
Va is the speed at which the stall speed curve (known as the 0-A) curve meets the positive g limit. The point at which this happens is called A (next significant point going clockwise is B, etc.) The main significance of this is that at or slower than Va, the aircraft will stall before you reach the positive g limit - so the main structure can't normally be overstressed.
A secondary significance of Va is that the structures people are then required to ensure that the structure of the flying controls are stressed for full deflection at any speed up to Va, and 1/3 of full deflection at Vne. Hence the fact that pilots are advised not to use more than 1/3 deflection above Va.
Normally, the stall speed curve 0-A follows the shape Vs=Vs1.N^½, so va will be Vs1 times the square root of the positive g limit. This isn't necessarily true for some more exotic flying machines such as flexwing microlights which show quite non-standard stall speed versus loading characteristics.
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