Is it "dressing" if I cooked my "stuffing" outside of the turkey?

The term 'stuffing' is used for the stuff that could go into a turkey, even if it never made it into a turkey. It might be more accurate to say dressing (but then, are the (sweet) potatoes or the cranberry sauce dressing too - and why one but not the other?), but most people would not bother with that fine a distinction.

Maybe this belongs on the Cooking? Or, maybe you'd get alternative answers there.

I think the difference between dressing and stuffing is mostly regional or dialect-related: in some places, if it's made of a starchy substance with flavorings, then it's stuffing regardless of whether it ever saw the inside of a bird, while in other places, it's dressing even though it was (under)cooked inside the turkey. However, there are certainly people and cookbooks (Joy of Cooking, for example) which make the distinction of stuffing=inside/dressing=outside.

As to why only this one dish type is called dressing, even though the sweet potatoes and cranberries and green beans appear to serve the same purpose, I couldn't tell you. Technically, if we were using dressing according to its usual definition (e.g. salad dressing), then the gravy should get this appellation.