Official Aviation Definition
Angle of Attack (AoA) is the aerodynamic angle formed between the chord line of a wing (the imaginary straight line connecting the leading edge to the trailing edge) and the vector representing the relative wind. If the angle exceeds the critical angle of attack, the airflow separates and aerodynamic stall occurs.
In Simple Words
The tilt of the wing compared to the direction the air is hitting it. Tilt it up too far, and the wing stops producing lift and stalls.
Real-World Flight Example
During a steep pull-up maneuver, the aircraft stick shaker activated as the angle of attack approached the critical 16-degree limit.