“But,” the whiners protest, “Bitcoin can’t be used as a unit of account! How can it be a currency when you can’t use it to distinguish a profit from a loss?”

To put this argument in context, let us review the three traditional functions of money. Money is often simultaneously a store of value, medium of exchange, and unit of account. The argument goes that if a good is not useful as a unit of account then it is not money. However, the only necessary relation between these three functions is that a medium of exchange must also be a store of value. A thing can serve as a store of value without being a medium of exchange or a unit of account, and a thing can be a unit of account without being either of the other two. Konrad Graf has written an article developing this point."

http://nakamotoinstitute.org/mempool/i-love-bitcoins-volatility/