Like other currencies, it has no "inherent value": its price is determined by how much people are willing to pay for it.
"It's not officially recognized, you can not pay taxes or use debits," says economist Garrick Hileman, a Cryptomanne researcher and professor at the University of Cambridge.
Some economists say it is a classic speculative bubble: euphoric investors paying for an asset much more than it is valid for fear of being left out.
They put the enthusiasm with bitcoin in the same category as the internet bubble of the year 2000 or the bubble in the American real estate market that led to the 2008 crisis.
"A lot of this is speculation, but there are signs that Bitcoin has indeed been used," says Hileman.