Center of Pressure
By definition, an airfoil produces lift when placed in an airstream. Different points along the airfoil produce lift in different amounts and in different directions. However, if you take the sum of all of these different force vectors, you end up with a single upward and a single downward force vector.
The points on the airfoil at which these force vectors are located is called the center of pressure.
The location of the center of pressure along the airfoil changes continuously with changes in angle of attack.
In a perfect world, these upward and downward force vectors would always be located the same distance away from the leading edge (that is, at the same chord length along the wing). If that was the case, there would be one and only one center of pressure.
Unfortunately, the world is not perfect and most of the time, the upward and downward force vectors are separated by some distance. When this happens, the airfoil tends to rotate. The tendency of the airfoil to rotate due to upward and downward forces located at different points along the airfoil is known as a pitching moment.
As the angle of attack changes, the centers of pressure will move and therefore the pitching moment experienced at any given point on the wing will change.
To put it another way, when the angle of attack changes, the total amount of lift and the rotational force felt by the wing at a given point both change.
However, there is one point on the wing, usually located at the one-quarter chord position, around which the pitching moment (rotational force) will never change - only the amount of lift changes.
The point at which the pitching moment remains constant with changes in angle of attack is called the aerodynamic center of the airfoil.