It's actually not, in regards to how the term is normally used for online gaming. The term is usually used for online currency that's strictly available within a given game and farmed up (there are several different ways) for use within that single game; BitCoin sounds like it was never attached to any game at all and was intended to be used as a currency replacement. (The Treasury was right, in that regard.)

And - at least in America - you never hear of online currency being exchangable for real money (in fact, all of the MMOs I know of specifically state that it isn't tranferable). In Japan and Korea, you'll sometimes see a fanstore with a "Purchase using gil/gold/creds/whathaveyou" special, but even then, it's very not common.