stalling occurs when a wing exceeds a critical Angle of Attack
That is the stock answer, the right answer, passes the written test.
But as a pilot, operating an aircraft, reading your POH, it says stall speed.
"references to "stall speed", apparently a speed below which a plane will stall"
How is this related to Angle of Attack, and why is stalling speed higher in a turn?
the real reason for stalling is that load force is more than the amount of lift force the wing can create under a given set of conditions. The pilot exceeds the critical Angle of Attack by pulling the stick back too far in an effort to create adequate lift.
For any given aircraft, the given set of conditions are:
Lift = Load (Mass × G force) = Density × Wing Area × Coefficient × V$^2$
Defining "Stall speed" based on Load covers all bases of flight.
Stall is defined in terms of speed for many GA aircraft because other variables are more or less constant and Angle of Attack is at its lift producing maximum
In level flight, at 1 G, the stall speed is, for example: 50 knots (flaps up).
Here, we may ask what is the "coefficient of lift"? This term is based on Angle of Attack and Airfoil type. When you lower your flaps, airfoil type changes, enabling increased lift without increasing aircraft AoA$^1$ or airspeed$^2$. Now, one can go slower before stalling, for example: stall speed 45 knots (flaps down).
Now for turns: stall speed 60 degree turn.
Load factor is 2 G. Coefficient of lift, density, and wing area are constant, velocity must be greater. You can read it from your POH, or calculate it as: V$^2$ = 2 × 50$^2$
V stall = 1.41 × 50 knots or 71 knots
Now for cases where Load is less than 1 G as previously discussed in current answers.
Aside from aerobatics, and an Aviation Stack Exchange favourite: lift is less than weight in a climb , we have the emergency descent. Yes, that's right, lift is also less than weight in a descent (as a mirror image of a climb) because now the drag component (rather than a thrust component) is helping resist gravity. This means Angle of Attack will be less than that required for level flight. Emergency descent turns are also performed at a higher airspeed, which additionally makes them safer.
$^1$ lowering flaps does increase local AoA at the wing. This is one reason that flaps are placed next to the fuselage, effectively increasing the "washout" of the wing tips. This ensures the warning buffet will start near the fuselage. The net effect is a lowering of stall speed (an increase in maximum lift for a given airspeed).
$^2$ drag also increases. Throttle must be added to maintain airspeed (or aircraft must lower its pitch to the horizon).