I am even more confused by your contentions now. You seem to be agreeing with me when I say that medium is pretty much a generic term to describe any of the numerous additives available to acrylic painters. This
is the proper way to describe mediums in this context. Here:
http://painting.about.com/od/artglossarym/g/defmedium.htm
Impastos, retardants, matting agents, glosses, fixers, thinners, glazes and so on are all mediums. It is a property added to an acrylic paint to modify its consistency to produce a certain effect.
But then you seem to contend that a medium is simply the solvent with binder piece of the 3 parts of a paint. While this is true, according to art glossary makes it clear that medium means more than this. Further, it still doesn't change that P3 Mixing Medium is a thinner. The manufacturer assesses it as such, and as a user of numerous thinners and other "mediums," I agree wholeheartedly
They certainly weren't meaning to describe their mixing medium as a solvent/binder. IOW, nobody is using their mixing medium to add pigment powders to it and make their own paint. Thus, my use of the terminology coincides with the intended use of it by P3.
If you follow the thread, I disagreed by saying that it is a thinner and not a Matt medium, as another poster suggested. This is when you interjected by disagreeing with me to say that it is a medium. This adds even more confusion to the debate, because it appears you were arguing for it to be a Matt medium, but then you qualified your definition to have a completely different, and irrelevant, meaning. Still, I enjoy a debate...