Double Spending

Double spending is the problem of using the same digital coin twice, and solving it is the reason blockchains exist at all. A digital file can be copied perfectly, so an electronic currency needs some way to prove that a specific unit has not already been spent somewhere else. Blockchains do this by broadcasting every transaction to a shared ledger and having nodes reject any attempt to spend an output that is already consumed. A double spend can still be attempted while a transaction is unconfirmed, or during a chain reorganisation that drops the block it was in. That is why merchants wait for confirmations: accepting a large zero-confirmation payment leaves the recipient exposed if a conflicting transaction gets mined instead.