What Is a 51% Attack and How Does It Specifically Enable Double-Spending?

A 51% attack occurs when an entity controls more than half of a blockchain network's mining hash rate. This control allows them to reorganize the transaction history and prevent new transactions from gaining confirmation.

Double-spending is achieved by sending a transaction to an exchange (e.g. selling coins) and then using the majority hash power to mine an alternative, private chain that excludes the original transaction, effectively 'undoing' the payment and retaining the coins.