double spend
An attack in which the same bitcoin is spent twice; Bitcoin prevents this through its confirmation and mining mechanism, which makes rewriting transaction history prohibitively expensive.
Before Bitcoin, digital cash systems faced a fundamental problem: a digital file representing value can be copied and sent to two different recipients simultaneously. This is the double-spend problem. Traditional solutions required a trusted central authority, such as a bank, to maintain a single authoritative ledger and reject duplicate payments.
Bitcoin solves this without any central authority. When a transaction is broadcast, nodes hold it in a waiting area called the mempool until a miner includes it in a block. Once the block is mined and added to the blockchain, the transaction receives its first confirmation. With each subsequent block, reversing that transaction would require redoing all the proof-of-work from that point forward while outpacing the rest of the network. This makes a successful double-spend attack economically impractical beyond a very small number of confirmations.
The prevention of double-spending was the core innovation that made Bitcoin possible. Every node independently verifies each transaction against the entire history of the blockchain, and the mining process makes fraud expensive rather than just forbidden. This combination replaced the need for institutional trust with mathematical certainty.