IMO, it's simple, unless you start saying everything is an RPG because you're playing the role of another character.
For me, RPG denotes a game where you have character progression through experience gains affecting abilities/attributes as you continue. From there, it branches out depending on the battle system. Is it turn based using menus? Is it real-time? So on and so forth.
By that definition, Far Cry 3 is an RPG. Is Devil May Cry? They don't call it experience there, it's red orbs or something instead but basically the same character progression.
Oh, what about Dark Souls when you do a SL1 run? Your character technically doesn't progress, so is it no longer an RPG?