It's a Bitcoin stored in your MtGox Account, as opposed to your own wallet or another exchange. MtGox make be experiencing solvency issues right now, and you a

It's a Bitcoin stored in your MtGox Account, as opposed to your own wallet or another exchange. MtGox make be experiencing solvency issues right now, and you are unable to freely transfer Bitcoins in and out of your account there. Some people believe that they might not actually exist, and the exchange will default, which is why those bitcoins are now worth less. About 2/3rds less. Previous to MtGox freezing transfers of Bitcoins in and out of your account, they had restrictions on the transfers of $USD in and out of your account - that resulted in the opposite situation, in which people who wanted to get their $USD out of the system, had to do so with bitcoins (which were, at that time, transferable) - they were therefore willing to pay MORE for bitcoins on MtGox, than other exchanges, just so they could somehow liquidate. This is why, historically, MtGox had a higher value for bitcoins - it was an artifact of customers betting that there was a solvency issue on the $USD in the system. Everything about MtGox is a train wreck right now. Useful (if expensive of some) lesson in the importance of how far you can trust a centralized exchange for this type of activity.