Except that CS:S and DoD:S are still technically mods; you can just buy them as standalone; but they still use an unmodified version of the Source engine (Unlike, say, SiN episodes, which is extensivly modifying the Source engine). Whenever Valve messes with the Source engine, it affects all of their source engine games because they're all mods of Half-Life 2.
Of course, the term "mod" isn't really defined. It was original a term for a game that "modifies" a preexisting game, and was pretty much coined during the Quake 1 era (Though Duke Nukem 3D and Doom both had what could be considered mods). Quake really set the definition thanks to the QuakeC scripting system which allowed for extensive modification of the game code without handing out the engine itself for free. Also unique was how Quake handled mods; they were left in seperate folders and activated by command lines, instead of having to whip up a batch file to covert game into the mod and back to the game when your done, like how Doom and Duke 3D handled it. Most mods of that time still used SOME data from the preexisting game (Textures and whatnot), though as time has gone on, mods have gotten more and more independant of the pre-existing games.
Basically, what defines a mod for me is the level of attachement to a pre-existing game. Mods like the original CS needed the Half-Life data files (And even the retail version included those files). Even standalone released like Gunman Chronicles used an unmodified version of hte Half-Life engine. They're both mods. This idea is carried into Half-Life 2 wherein even if you buy a mod seperately, you still get the same set of base Source engine files. (You just don't get, like, the maps for HL2, because CS:S doesn't need them)
The UT2004 version of Red Orchestra was a mod; it required UT2004 to run, period. This upcoming game, however, is NOT a mod. It's a completely independant, standalone game made with the Unreal engine. It's literally no more a mod than Gears of War or Wheel of Time, or any other unreal engine game. Is it based upon a franchise that was originally a mod? Yes, but that doesn't make this a mod. It's a fully licensed seperate game and the RO guys are at liberty to make any modifications they want to the engine since it's not a mod and doesn't need to use another games files to work.
