I guess you will not get any answer here that can be used as "definition". The term RPG was once an umbrella where you could throw some games in. Like Baldurs Gate, Fallout 1, and many older titles. But even there you had distinctions. Its clear that Dagerfall was a different kind of RPG then lets say Wasteland or some of the DnD games.
To make the situation worse though, today pretty much anything can be sold as "RPG" where the term "action RPG" has become the most popular term. Because any game even if it is not an RPG can be sold as "action" RPG. Like Fallout 3 and Oblivion - which are in my eyes no RPGs at least.
I personally think for an RPG the interaction between the character and the world is very important. Take Oblivion as example. What ever you do is not important. The world has pretty much no reaction to it.
To make the situation worse though, today pretty much anything can be sold as "RPG" where the term "action RPG" has become the most popular term. Because any game even if it is not an RPG can be sold as "action" RPG. Like Fallout 3 and Oblivion - which are in my eyes no RPGs at least.
I personally think for an RPG the interaction between the character and the world is very important. Take Oblivion as example. What ever you do is not important. The world has pretty much no reaction to it.