According to the Austrian School, money is "the most sought after good". Whatever the good is, in this case Bitcoin, it is "most sought after" because every actor in the market knows that this good, can be passed off to the next guy if needed for exchange of goods, and the next guy will accept it at the same value. In order for this exchange to occur, it helps for the most sought after good to possess certain characteristics.
It will be fungible, divisible, scare, portable, and have a high value to weight ratio. We don't use chicken eggs, panes of glass, gasoline, and sand as money, hopefully for obvious reasons when we consider the 5 characteristics of good monies.
Legal tender laws often force us to accept a certain item as money even though in a free market we would choose another good as money. The Federal Reserve bank note being an example, it can be created at will which means it is not scarce. Something that can be counterfeited makes for a horrible currency, whatever other "benefits" it may bring.
Bitcoin has the characteristics of good money. Arguably it is better than gold as money because of its portability, divisibility, and ease of transfer. However bitcoin does not have widespread recognition or adoption. It is used as money, but it's use is widespread.
This does not reduce bitcoins money-ness any more than the fact that Pesos are still money even though they aren't used by 99% of the world.
Paul Krugman and his ilk will suggest that using inflatable toilet paper notes as money is a good thing. That money can only be money if it is backed by government and forced onto individuals.
This is wrong. Money will arise any time exchange takes place between humans.
Left to its own operations, meaning without the guns of government interfering, the free market will eventually agree on a form of money.
Money allows humans to escape from barter trade because of the phenomenon called the "coincidence of wants". Money is necessary to the division of labor, therefore money is also necessary for the advancement of economies beyond barter.
If what I wrote doesn't make sense then I suggest you check out the book store and library at mises.org
They have many free e-books.
So does FEE.org
http://www.fee.org/library/