Spending an amount of currency twice in a system of digital cash in a fiat system such as the Euro or a cryptocurrency such as Bitcoin. Digital transactions can be falsified if there is no protective mechanism in place.
Crypto Double Spending
With regard to Bitcoin, in order to double spend, a miner has to create an erroneous block faster than the next valid block. The miner has to introduce the fake block into the network before the fraud is caught so that it has time to return any spent crypto in order to use it again. Double spending was addressed in Satoshi Nakamoto's initial abstract describing Bitcoin (read the end of
Satoshi Nakamoto).