Started to post this in that other thread, but since it is more looking for a technical definition of RPG, I rather think it's more suited here...
If I were to explain what an RPG is, I wouldn't use a technical approach, but rather an exemplaric one.
For example, I would first find out a fantasy movie or fantasy novel everyone involved knows (the Lord of the Rings is a good choice nowadays).
Then I would explain how fun it is to watch the movie, but at some points you always start to think about what you would have done differently in the same situation, i.e. wouldn't you want to push Isildur into Mount Doom together with the One Ring, if you were Elrond back then!?
Now, if you were able to decide, the whole course the story is taking might be different and must be changed accordingly.
That's the basic principle of roleplaying, a story is told by several people together and the outcome determined by the actions these people take.
It's not an old story told again, but rather an entirely new story told from the beginning. A story that is created during the game, as if it were happening right now and you were right in the middle of it.
Then I would start to explain what a player character is, what a game master is, and so on...
Bye
Thanee