I also prefer 'Opacity' since it's a more artists-friendly term. However, we are already quite internally inconsistent in Blender, mixing the use of Alpha and Opacity. In computer graphics, 'alpha' is generally used more I think. Adding @Brecht Van Lommel (brecht) as reviewer here - hopefully we can either be consistent or at least formulate some kind of clear guideline on when to use Alpha vs Opacity.

I think opacity is fine here. I would reserve the term alpha for cases where we are specifically referring to the alpha channel of an image, and opacity/transparency for everything else. But I wouldn't prioritize the work required to implement that everywhere.