So I was thinking about this. The definition of a Role Playing Game is, from Wikipedia: "a game in which players assume the roles of characters in a fictional setting. Players take responsibility for acting out these roles within a narrative, either through literal acting or through a process of structured decision-making or character development^. Actions taken within many games succeed or fail according to a formal system of rules and guidelines".

More specifically, a videogame RPG is "a video game genre where the player controls the actions of a main character (or several adventuring party members) immersed in some well-defined world".

Basically, an RPG is a game where the player/s act as a fictional character in a fictional setting with a fictional narrative. That seems to fit very well with the traditional RPG cases (FF, Pokemon, TES, DQ...). But doesn't this definition include a lot of other games? In CoD you are a soldier who fights against a series of antagonists to save the world. In Tony Hawk you are a skateboarder that wants to become the best skateboarder, in Age of Empires you are the leader of a country, and you have to develop your side and defeat your enemies...There are more examples, but you get the point. Aren't most videogames RPGs by these definitions? Obviously, if you look at a DQ game and a Metal Geat game, most people would say DQ is the RPG, but only because most of us have clear ideas and preconceptions of what a RPG must be, even if we don't really know how to define them.

What defines an RPG and distinguish it from the rest of the videogame genres?

Bingo. Also stuff like classes.

From developers standpoint. It is a way of designing the different elements and concepts in the game to scale and adapt with / against each other. For example, the "attack power" stat / level of your character scales with the "defense" stat of the enemy to determine the amount of damage caused, and so on.

An essential part of that design is to give the player a direct, in-depth control over those stats and numbers ( level / equipment / spells...etc), so he can determine how things "interact" with each other. That way, the player will play the main "role" in the experience.

True in a sense but I wouldn't call COD a rpg or a hybrid. Many genres of games have rpg elements now. It adds a lot more depth and replayability to a game that wasn't there before. 

I used the CoD example for the single player. I suppose every game with a campaign or a story mode is going to have the most basic RPG elements (a fictional setting, a protagonist and a some elements to advance the story), but that is kind of the point of this thread, to difference the RPGs from the rest of the games.

Basically. You can customize the perks, gear and weapon mods of your loadout. There's no actual character stats. The single player is just like any other FPS with no rpg elements.

The modern definition in video games, given the number of hybrid genres that came up in the recent years, would be the presence of an evolving character sheet.

It's a Role-Playing Game ... (OK that would only make people roll their eyes.)

"Role-playing" has a very specific definition in this case and playing just any role doesn't count cause that practically breaks the idea of an RPG ... 

Role-playing in the case of RPGs would most likely mean being able to "assume multiple distinct roles that have their own specific purpose and skills" ... 

A game with a class system that have differing purposes and abilities would be classified as an RPG. I believe this is the ORIGINAL definition of an RPG when it was defined as such for tabletop RPGs ...

If you look at the division and compare it to halo, COD or GTA, the differences become more obvious between RPG and other genres.