Double Spending

Definition

Double spending is an attempt to use the same digital funds in conflicting transactions. Bitcoin prevents both transactions from being valid within one accepted history when they try to spend the same previously unspent output. Nodes check that an output exists, that its spending conditions are met, and that it has not already been consumed. However, conflicting transactions can circulate before confirmation, and competing blockchain branches can temporarily include different spends. Proof of work and confirmation depth make replacing an accepted history progressively more difficult under the network's security assumptions. Merely seeing a transaction broadcast is therefore weaker evidence than verifying its inclusion and subsequent confirmations. Double spending does not require copying a physical coin or learning someone else's private key; it concerns disagreement over which authorized spend becomes part of the ledger that recipients ultimately rely on.