What exactly is meant by "RPG theory?" Does anyone know who coined it? Are there any sources defining it?
To provide some further context, the phrase film theory is quite common.
Film theory is an academic discipline that aims to explore the essence of the cinema and provides conceptual frameworks for understanding film's relationship to reality, the other arts, individual viewers, and society at large. Film theory is not to be confused with general film criticism, though there can be some crossover between the two disciplines.
RPG theory is pretty much exactly the same thing, but with a different media the target of scrutiny. And of course, it doesn't have as much academic grounding at this point in history -- we can hope that changes over time!
The concept of RPG theory exists completely separate from any particular model, such as GNS.
The Wikipedia entry on Role-playing game theory is rather comprehensive. It makes note of the Threefold Model, GEN Theory, The Big Model, Color Theory, Channel Theory, The Turku School, and The Meilahti School, each of which is an example of a particular theory of roleplaying games.
The page leads with this definition of RPG theory:
A role-playing game theory is an academic or critical study of role-playing games (RPGs) as a social or artistic phenomenon. RPG theories seek to understand what role-playing games are, how they function, and how the process can be refined in order to improve the gaming experience and produce more useful game products.
I don't think someone really coined the term, for RPG Theory is, well... the theory that stands behind roleplay games. No less and no more.
Whenever you don't talk about a game but you talk about how a game has benn made, why some design choices have been taken or whenever you look for some ways to analyze games, you fall into the game theory field.
I guess that's what theory is all about.
As for the sources:
The most famous attempt to formalize some RPG related theory is the now-closed forum the Forge.
Here is where some universally recognizable (but not universally recognized) theoretical schematizations like the GNS model or the Big Model were born.