Double-spending is a failure mode of digital cash schemes, when it is possible to spend a single digital token twice. Since, unlike physical token money such as coins, electronic files can be duplicated, and hence the act of spending a digital coin does not remove its data from the ownership of the original holder, some other means are needed to prevent double-spending.
The cryptocurrencyBitcoin implemented a solution in early 2009. It uses a scheme called proof-of-work, to avoid the need for a trusted third party to timestamp transactions. These timestamps are recorded in its public ledger called the block chain. This avoids anyone double-spending the currency.
^Osipkov, I.; Vasserman, E. Y.; Hopper, N.; Kim, Y. (2007). "Combating Double-Spending Using Cooperative P2P Systems". p. 41. doi:10.1109/ICDCS.2007.91.
^Joshua Kopstein (12 December 2013). "The Mission to Decentralize the Internet". The New Yorker. Retrieved 30 December 2014. The network’s "nodes"—users running the bitcoin software on their computers—collectively check the integrity of other nodes to ensure that no one spends the same coins twice. All transactions are published on a shared public ledger, called the "block chain"