Bitcoin is a cryptocurrency invented in 2008 by an unknown person or group of people using the name Satoshi Nakamoto and started in 2009 .
There are two key traits of Bitcoin: it is digital and it is seen as an alternative currency.
Unlike the notes or coins in your pocket, it largely exists online. Although there are some specialist ATMs which issue bitcoins, it may be best to think of them as being more like virtual tokens.
And secondly, Bitcoin is not printed by governments or traditional banks.
That means it is "not legal tender, you can't pay your taxes or use it to settle debts", says Dr Garrick Hileman of the Judge Business School at University of Cambridge.