How is wing area defined?
Yes, the plain wing area is also used for wing loading. How the value is calculaed depends on the method used. In all cases the center section and ailerons are included but the horizontal tail is excluded. Common is also the use of the projected area of the clean wing in the x-y-plane, because that is the area relevant for lift.

For airliners, the two most important methods are the Wimpress and the Airbus method (the Wimpress being used by Boeing). The difference is the triangles you get by extending the wing's leading and trailing edges forward into the fuselage for Wimpress versus connecting the points where the leading and trailing edges meet the fuselage on both sides by two straight lines for Airbus. This discussion on airliners.net lists a comparison of the wing areas you get with both methods for a range of models.



Wing area methods in comparison. The shaded are is covered by the fuselage and determined differently; creating different areas for the same wing.

Note that the specifics of the method used is irrelevant for the purpose: This wing area serves as the reference area of most aerodynamic coefficients and either method is good enough for the purpose. Consistency is more important than precision here.

Both methods have been around for a while, so there was no change recently. But since both are used in parallel, there is no common standard.