Double Spend

A double spend is paying two people with the same money. With paper cash it's impossible: hand over a note and it's gone from your pocket. But digital things can be copied, so a digital coin needs something to stop you sending the same one twice.

Banks solve it with a central ledger. Bitcoin's big breakthrough was solving it without one: thousands of nodes agree on a single shared history, and once a payment is buried under enough blocks it's practically impossible to undo. A 51% attack is one way to try; accepting a zero-confirmation payment is another way to get fooled.