Double-spending is spending the same digital coin more than once. Cash changes hands. Files copy. If the network cannot mark a coin spent, the money is broken. Banks stop this with a central ledger. Decentralized networks use consensus instead. Proof of Work and Proof of Stake make rewriting history expensive.
Each new spend is checked against the ledger. Once confirmed, those coins are spent and cannot be reused. Stopping double-spending is the problem Bitcoin solved and the reason a blockchain exists. Without it, decentralized digital money does not work. Unlike physical cash, which must be handed over, digital data can be copied.
If a network cannot verify that a coin has already been spent, the currency's integrity collapses. Traditional banks solve this with a central database that logs every debit and credit. Proof-of-work and proof-of-stake require participants to invest computational effort or stake assets, making it prohibitively expensive to rewrite transaction history.
If two spends of the same coin can both look valid, digital cash fails. Consensus picks one history.
Double-Spending Attack
Attempting to spend the same digital currency twice.