Double Spend
Spending the same crypto twice by getting two conflicting transactions accepted, the core problem blockchains were built to stop.
Digital money is just data, and data can be copied. Without a referee, nothing stops you from sending the same coin to two people. Banks solve this with a central ledger. Bitcoin solved it without one — its main breakthrough.
The blockchain orders transactions so the network agrees which came first; the second, conflicting spend is rejected. This is why merchants wait for confirmations. A payment with zero confirmations could still be reversed by a fast attacker or, on a weak chain, a 51% attack.
Related terms
An attack where one party controls most of a network's mining power and can rewrite recent transactions.
ConfirmationEach new block added on top of the one holding your transaction, deepening it and making reversal far harder.
BlockchainA shared digital ledger that records transactions in linked batches, copied across many computers and hard to alter.
TransactionA signed instruction that moves value or data on a blockchain, recorded permanently once miners or validators confirm it.