All these cryptocurrencies are made possible by an ingenious solution to a problem which would otherwise make purely digital currencies impossible. A digital currency is one which exists solely as a string of numbers inside a computer and copying numbers at lightning speed is the core competence of any computer. So there seemed to be no way to stop any given piece of digital currency from being copied and spent unlimited times. It seemed to make digital money impossible even in theory. Applied to money, this means that the same bitcoin can’t be spent twice without changing hands.

Source: The Guardian July 23, 2017 17:26 UTC