What is an RPG?

I recently (re-)listened to Active Time Babble episode V, which consists mostly of a discussion of the definition of the genre “RPG.” As is usually the case with this particular topic, there was no real definitive conclusion; but I think I might have stumbled on one for myself:

A role-playing game models the development of character skill as distinct from player skill such that the manipulation of this model is the primary expression of player skill.