Definition
A double spend is the act of successfully spending the same coins more than once. It is the central problem any digital cash system must solve, because unlike a physical coin, digital data can be copied. Bitcoin's entire architecture of blocks, mining, and proof-of-work exists to produce one shared transaction history that is computationally impractical to rewrite, so a coin can only be spent once.
How Bitcoin prevents it
Nodes reach consensus on a single ordering of transactions. Once a transaction is buried under confirmations, replacing it would require re-mining every block on top of it and out-pacing the honest network, which grows exponentially harder with depth. There is no central authority; the cost of rewriting history is what enforces the rule.
Attack windows
The realistic exposure is on transactions with zero or few confirmations. A race attack broadcasts a conflicting transaction to the network at almost the same time as the merchant's copy, hoping the conflicting one gets mined. A Finney attack uses a miner who withholds a pre-mined block containing a conflicting transaction, then releases it after receiving goods. Both target unconfirmed or single-confirmation payments.
Waiting for additional confirmations is the standard defense, and a successful large-scale double spend would require the deep reorg that only a 51% attack can produce. For everyday payments this risk is negligible once a transaction is a few blocks deep.
In Simple Terms
A double spend is the act of successfully spending the same coins more than once. It is the central problem any digital cash system must…